Complete Guide to Prescription and Over the Counter Diet Pills

Assortment of pills, tablets, and capsules


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Are you thinking about using an over-the-counter weight loss pill? Or perhaps you are interested in a diet pill that requires a prescription. Deciding whether a weight loss pill, prescription, or supplement is right (and safe) for you requires understanding how they work, their side effects, and effectiveness.

The best resource for information regarding the use of any supplement or weight loss pill is your healthcare provider. They will also be able to discuss how taking a diet pill might interact with your other medications and provide the best advice regarding the safety of certain products.

Types of Weight Loss Pills 

There are two different types of diet pills. They are prescription weight loss pills and over-the-counter (OTC) weight loss pills, the latter of which also include herbal supplements.

Prescription weight loss pills are medications that you get through your doctor. OTC diet pills and herbal weight loss supplements can be purchased without a prescription and are available in many vitamin shops and drugstores.

Generally speaking, both prescription and OTC weight loss pills have one of three goals:

  • To reduce your appetite, making it easier to make healthy food choices
  • To increase your body's ability to burn fat, creating a larger calorie deficit
  • To inhibit your body's ability to absorb the fat in the foods you eat

FDA Approval

Some diet pills are approved by the U.S. Food and Drug Administration (FDA). Others don't require FDA approval. This chart outlines what it means to have FDA approval, as well as what it means if a diet pill is not approved by the FDA.

FDA-Approved Drugs
  • Proven to be safe

  • Proven to be effective

  • Meets federal quality standards

  • Benefits outweigh the risks

Drugs Not Approved by the FDA
  • Safety not reviewed or approved

  • Effectiveness not reviewed or approved

  • Claims of what the product can do are not approved or verified

It is also important to realize that some diet pills contain ingredients not listed on their labels which are also not approved by the FDA.

A 2018 study looked at 317 different weight loss products and found that 269 of them (84.9%) contained sibutramine—an ingredient that the FDA removed in 2010—as a hidden ingredient. Though this substance helps reduce appetite short term, it has also been connected with mood changes, increased blood pressure and heart rate, and even amnesia.

Other hidden ingredients found in some of the weight loss pills studied were:

  • Phenolphthalein, a laxative that the FDA removed in 1999
  • Fluoxetine, a prescription antidepressant also known as Prozac
  • Ephedra, a substance banned in dietary supplements since 2004
  • Belviq, a drug the FDA requested be removed from the market in 2020

The availability of diet pills via the internet has made these substances even harder to regulate and control. It also makes it more difficult to determine whether the product you buy has safe dosage levels, potentially increasing your risk of taking a toxic amount.

Herbal supplements are not considered medications and, therefore, do not have to follow the strict safety guidelines that govern medicines.

Prescription Diet Pills 

The U.S. Food and Drug Administration (FDA) has approved a range of medications designed to help with weight loss. These use different mechanisms and may only be appropriate for certain people.

Xenical

How it works: Approved by the FDA in 1999, Xenical (orlistat) is a lipase inhibitor. This means it works by decreasing the absorption of fat, which the body then eliminates in the stool.

Effectiveness: One study of 400 people taking 120 mg of orlistat three times per day reported that users significantly reduced their weight, with 27.4% losing at least 5% of their beginning weight. A 2018 review found that the average weight loss with orlistat is 2.9 kg (6.4 pounds).

Side effects: The most common side effects are gastrointestinal (loose stools, stomach pain) from undigested fat in stools, especially if a low-fat diet is not followed. Other potential effects include headache, anxiety, and irregular menstrual periods.

Who may benefit: This weight loss drug is often prescribed for people who are overweight and might also have other medical conditions, such as high blood pressure or cholesterol, diabetes, or heart disease.

Risks: Orlistat may cause damage to the liver and/or kidneys. It also blocks the absorption of beta-carotene and some of the fat-soluble vitamins, making a multivitamin necessary to ensure that your body gets enough of these nutrients.

Qsymia

How it works: Qsymia contains two substances: phentermine and topiramate. Phentermine (which is released immediately) impacts food intake by increasing norepinephrine in the body. Topiramate (which is released later in the day) is thought to suppress appetite and increase energy. 

Effectiveness: Both phase I and phase II trials have found that the phentermine-topiramate combination aids in weight loss, with the average loss of roughly 10% of starting body weight.

Side effects: Actual effects can vary depending on the dosage prescribed, with some of the most common being dry mouth, constipation, tingling or prickling of the skin, insomnia, irritability, and changes in taste. Some people also experience headaches, insomnia, depression, anxiety, fatigue, and blurred vision.

Who may benefit: Your physician may prescribe Qsymia if you have a body mass index (BMI) over 30, or a BMI of 27 or higher along with a weight-related condition such as type 2 diabetes or high blood pressure.

Risks: Exposure to this drug combination during pregnancy may increase the risk of the baby being born with cleft lip or cleft palate.

Saxenda

How it works: Saxenda (liraglutide), is an injectable medication that reduces food intake by decreasing appetite and increasing feelings of fullness. 

Effectiveness: A 2016 review found that, after one year, the average user lost between 8.9 pounds and 13.3 pounds. Another study noted that almost two-thirds of users decreased their body weight by 5% or more, one-third lost at least 10% of their body weight, and 14 out of 100 participants lost 15% of their body weight.

Side effects: Gastrointestinal side effects (nausea, diarrhea, constipation) are the most common with Saxenda, especially when first taking the drug. Other potential effects include headaches and low blood sugar.

Who may benefit: Saxenda is designed to be used by people who are obese (with a BMI of 30 or more) or by people who have a BMI of 27 or more and a weight-related medical condition such as type 2 diabetes, high cholesterol, or high blood pressure.

Risks: Gallbladder disease, acute pancreatitis, and severe hypoglycemia are a few safety concerns associated with Saxenda. This drug shouldn't be taken by people who take insulin, people who are breastfeeding, or those with a personal or family history of medullary thyroid cancer or multiple endocrine neoplasia syndrome type 2.

Contrave

How it works: Contrave (a combination of naltrexone hydrochloride and bupropion hydrochloride) affects the central nervous system to increase the number of calories burned and reduce appetite. This pill is often prescribed along with a reduced-calorie diet and exercise program.

Effectiveness: Research indicates that people taking Contrave lose more weight than those taking a placebo, with a mean loss of 5.4% of initial body weight, and they typically maintain that loss for at least 56 weeks.

Side effects: The most common side effects are nausea, vomiting, diarrhea or constipation, headache, dizziness, insomnia, and dry mouth. The buproprion in Contrave can also cause suicidal thoughts or actions, particularly when first taking it.

Who may benefit: Contrave is designed for people with a BMI of 30 or higher, or those with a BMI of 27 and up with another weight-related medical condition.

Risks: Contrave may increase your risk of seizures, increased blood pressure or heart rate, low blood sugar, liver damage, visual issues, and manic episodes.

Phentermine

How it works: Phentermine—which is marketed under a long list of names, including Suprenza, Adipex-P, and Lomaira—works by decreasing appetite.

Effectiveness: A 2013 controlled trial reported that 43.3% of subjects taking 7.5 mg phentermine lost at least 5% of their body weight, with 46.2% of those taking 15 mg achieving the same effect. A 2019 study adds that taking this drug for three months or more can provide greater weight loss.

Side effects: Side effects can include insomnia, diarrhea or constipation, vomiting, an unpleasant taste, and dry mouth.

Who may benefit: Like many of the other prescription weight-loss drugs, phentermine is designed for people with a BMI of 30 or higher, or those with a BMI of 27 who have other weight-related medical conditions.

Risks: This drug should not be used by people who are pregnant or nursing, or those with cardiovascular disease, hyperthyroidism, glaucoma, or a history of drug abuse.

Wegovy

How it works: Wegovy, a weekly injection of semaglutide, works by reducing appetite, decreasing calorie intake. It activates areas of the brain responsible for appetite regulations and food intake.

Effectiveness: Trials showed that treatment with Wegovy produced a statistically significant reduction in body weight over the course of 68 weeks. Subjects lost 14.9% of body weight compared to 2.4% with placebo.

Side effects: The most common side effects of Wegovy include nausea, diarrhea, vomiting, constipation, abdominal pain, headache, fatigue, dyspepsia, dizziness, abdominal distention, eructation, hypoglycemia in patients with type 2 diabetes, flatulence, gastroenteritis, and gastroesophageal reflux disease. It can also cause hypersensitivities such as anaphylaxis, retinopathy in diabetics, heart palpitations, or a racing heartbeat. Suicidal ideation is also a potential side effect, as seen with other weight loss medications.

Who may benefit: Wegovy can benefit adults with a starting body mass index (BMI) of greater than 30 (obesity) or greater than 27 (overweight) who have at least one weight-related health condition such as hypertension, type 2 diabetes mellitus, or dyslipidemia.

Risks: This drug should not be used by people who are pregnant or who are planning to become pregnant. Personal or family history of MTC or patients with MEN 2 should not take Wegovy. Nor should patients who have had a previous hypersensitivity reaction to semaglutide or any of the ingredients in Wegovy.

Over-the-Counter Diet Pills and Supplements

When purchasing a non-prescription weight loss aid, it's important to know that while OTC medications are FDA-approved for safety and effectiveness, weight loss supplements are not. It's the responsibility of supplement manufacturers to ensure that these products are safe.

For this reason, it is important to be careful when buying a diet supplement or popular herbal supplement for weight loss. The FDA also does not approve most of the claims that companies make about their products. In many cases, weight loss claims are carefully crafted to make the product sound more effective than it is.

Alli

How it works: The only over-the-counter weight loss pill approved by the FDA, Alli, contains a lower dose of orlistat (60 mg), the same medication that is in Xenical. It works by reducing the body's absorption of fat.

Effectiveness: One review of 33 randomized controlled trials involving 9,732 participants reports that orlistat provides a "slight but significant decrease in body weight." A 2014 assessment indicates that the amount of weight lost at the one-year mark is around 3 kilograms (6.6 pounds) when combined with a low-calorie diet.

Side effects: People who take Alli may experience uncomfortable gastrointestinal side effects, which can be minimized by following a less than 30% fat diet.

Who may benefit: This OTC weight loss drug is for adults with a BMI of 25 or above and should be combined with a low-calorie, low-fat diet.

Risks: Alli should not be used by people who are pregnant or breastfeeding, people who have had an organ transplant, those who've been diagnosed with food absorption issues, or those taking cyclosporine. Orlistat may increase the risk of kidney damage and osteoporosis, and it may also interact with some drugs, such as warfarin and antiretroviral medications.

Alli was voluntarily removed from the market in 2014 after a tampering scare, but the company re-released it with a tamper-evident package in 2015.

Garcinia Cambogia

How it works: Garcinia cambogia is derived from a fruit that grows in warmer climates and is said to assist with weight loss by reducing appetite.

Effectiveness: Claims made by sellers have not been supported by scientific research. There is little evidence to support its effectiveness. A review published in 2015 reports that results are mixed—some people appear to lose weight with this substance while others do not.

Side effects: Adverse effects associated with taking garcinia cambogia can include headache, nausea, diarrhea, and other gastrointestinal issues.

Who may benefit: Studies that have found garcinia cambogia effective for weight loss report that this effect is the same regardless of age, sex, or any other weight-related conditions. It may be better at reducing visceral fat (belly fat) than some other substances.

Risks: While rare, the use of this substance may result in liver damage, and it is unclear whether it is safe for people who are pregnant or breastfeeding.

Glucomannan

How it works: The name of this diet supplement may not sound familiar, but you've probably seen products that contain glucomannan, which is basically a fiber supplement. Lipozene is one.

Effectiveness: In 2020, authors of a review study reported that they could not confirm that this fiber substance can actually promote weight loss.

Side effects: People taking glucomannan may notice that they belch more often and feel bloated, as well as having feelings of stomach fullness—at least for the first few days. Diarrhea and constipation might also occur, particularly when taken in higher doses.

Who may benefit: Some studies that connect glucomannan with weight loss show more positive findings in female subjects, with even greater effects when this supplement is combined with garcinia cambogia.

Risks: The EFSA Panel on Food Additives and Nutrient Sources states that this supplement presents no concern for being toxic or allergenic as long as dosing stays below 3 grams per day.

On January 10, 2020, the FDA announced that glucomannan would be added to the definition of dietary fiber (after receiving a petition from a citizen), enabling it to be included on products' Nutrition and Supplement Facts labels.

Conjugated Linoleic Acid (CLA)

How it works: Conjugated linoleic acid (CLA) occurs naturally in meat, dairy, and vegetable oil and is thought to assist with weight loss, in part, by reducing the body's absorption of fat.

Effectiveness: A 2016 review of seven studies concluded that CLA might promote weight loss, but the difference in taking this substance versus taking a placebo is relatively small. Another 2016 study found that some people who took a CLA supplement experienced increased insulin resistance and lower HDL cholesterol levels.

Side effects: Potential side effects include constipation, diarrhea, soft stools, and abdominal pain.

Who may benefit: CLA has been found to provide anti-inflammatory effects and improved lipid profiles.

Risks: Some people have experienced a myocardial infarction (heart attack) after taking CLA, so this supplement might not be advised for people with heart issues.

Raspberry Ketones

How it works: Raspberry ketones come from red raspberries (it gives them their smell). The claim is that the product can reduce weight by breaking down and reducing fat storage, and increasing metabolism.

Effectiveness: This weight loss supplement became popular after Dr. Oz mentioned it on his TV show, but there is not enough evidence to support the claims that raspberry ketones can help humans lose weight.

Side effects: People taking raspberry ketones may feel jittery and have increased blood pressure and heart rate.

Who may benefit: It is currently unclear whether this dietary supplement offers any real health benefits. Plus, many products contain a much higher amount than has been determined safe (100 mg per day).

Risks: Research suggests that raspberry ketones may be associated with coronary vasospasm, which is when the coronary arteries suddenly constrict and reduce blood flow to the heart. Additional studies indicate that this supplement may interfere with the medication warfarin and be unsafe for people who are pregnant or lactating.

Forskolin

How it works: Forskolin is an extract from the coleus forskohlii plant advertised as a diet supplement, carb blocker, and fat burner.

Effectiveness: Some studies have found that forskolin may help with weight loss, whereas others have found it doesn't, so its effectiveness is not yet proven.

Side effects: Side effects can include increased bowel movements and loose stool and generally reduce over time.

Who may benefit: One small study found that people taking forskolin had increased HDL cholesterol levels (the "good" cholesterol) and improvements in their insulin concentration and resistance when combined with a low-calorie diet.

Risks: This supplement may pose risks for people with polycystic kidney disease.

Meratrim

How it works: Meratrim is a mixture of extracts taken from the flower heads of the Sphaeranthus indicus plant and the fruit rind from Garcinia mangostana and is thought to help reduce weight by reducing appetite.

Effectiveness: A study of 60 participants with a mean BMI of 28.3 found that those taking Meratrim had greater reductions in their weight as well as in the size of their waist and hips.

Side effects: While some study participants did experience nausea, stomach pain or discomfort, and gastritis, researchers concluded that these effects were not related to Meratrim use because the placebo group also experienced effects. In a 2013 study, some participants experienced headaches, nausea, gastrointestinal issues, and various body pains, but, again, these were not attributed to Meratrim since the placebo group experienced them too.

Who may benefit: The 60-participant study also reported that those taking Meratrim had "significant" changes in their LDL cholesterol (the "bad" cholesterol), triglycerides, and total cholesterol.

Risks: Research indicates that, while the risk appears to be small, Garcinia-based supplements may be linked to liver damage, serotonin toxicity, and mania.

Chromium

How it works: Sometimes referred to as chromium picolinate, products that contain this substance often claim to help improve calorie burn and decrease appetite.

Effectiveness: While some research has shown a small correlation between chromium and weight loss, there is insufficient evidence to support a more substantial claim.

Side effects: In some trials, people taking chromium have reported watery stools, constipation, nausea, vomiting, headache, vertigo, and hives.

Who may benefit: Chromium is considered an essential nutrient and may have some antioxidant effects.

Risks: Chromium is generally considered safe as long as it is not consumed in high amounts. It may interact with some medications, such as insulin, antidiabetes medicines, and some hypothyroidism medications.

Green Tea

How it works: Green tea can be consumed as a beverage or in pill form and claims to increase metabolism, resulting in weight loss.

Effectiveness: While green tea is safe when consumed in moderation, little evidence supports its use as a long-term weight loss supplement.

Side effects: Green tea as a beverage is generally well-tolerated, but people taking green tea extract may experience constipation, nausea, abdominal discomfort, and increased blood pressure.

Who may benefit: Green tea is rich in polyphenols and may provide anti-inflammatory, antioxidant, and anti-cardiovascular effects.

Risks: Some researchers have expressed concerns that green tea extract may cause liver damage.

Green Coffee Bean Extract

How it works: Green coffee is another name for raw coffee beans (so this type of product does contain caffeine). It is thought to inhibit the body's storage of fat and/or reduce appetite.

Effectiveness: A study of 64 obese women found that while they all lost weight on a low-calorie diet, those who received 400 mg of green coffee bean extract for eight weeks lost more weight than those who did not take the extract. They also had lower total cholesterol and LDL, or "bad" cholesterol levels.

Side effects: Reported adverse effects include headache and urinary tract infections.

Who may benefit: One review concluded that green coffee extract could improve fasting blood glucose levels, insulin levels, and total cholesterol.

Risks: Consuming 200 mg of green coffee bean extract per day is generally considered safe, though this has not been adequately studied.

Hoodia

How it works: This herb is extracted from a flowering cactus plant (hoodia gordonii) and sold as an appetite suppressant. It can be consumed in tablet, pill, or powder form.

Effectiveness: A 2021 review reports there is insufficient scientific evidence to support the claims that hoodia is an effective form of long-term weight management.

Side effects: Some people taking hoodia have experienced nausea, vomiting, dizziness, and strange skin sensations.

Who may benefit: Hoodia may provide positive effects for people with metabolic diseases.

Risks: Hoodia may be unsafe for some people due to its effects on blood pressure, bilirubin, and heart function.

Bee Pollen

How it works: Bee pollen has been found to speed up fat metabolism, thus making it an appealing weight loss supplement.

Effectiveness: There is little evidence supporting bee pollen's use for weight loss, particularly because many studies have been conducted on animals versus humans.

Side effects: The main side effect concern with bee pollen is if you are allergic, which can result in hives, wheezing, and anaphylaxis—especially if you've had a serious reaction to bee stings.

Who may benefit: Bee pollen has several beneficial health properties, including being an anti-inflammatory, antimicrobial, and antifungal substance.

Risks: In 2014, the FDA flagged the company Oasis Bee Pollen for making false and misleading claims about its product's weight loss effects. The FDA also warned of hidden ingredients in the company's supplement, including sibutramine, which can increase blood pressure, and phenolphthalein, which can cause cancer.

If the over-the-counter diet pills or weight loss supplements you're interested in are not listed above, visit the National Institutes of Health Office of Dietary Supplements. The NIH provides a comprehensive list of diet supplements and current information about the safety and effectiveness of each one.

Banned Weight Loss Drugs

There are a few drugs that the FDA has banned for consumer use.

Ephedra

After ephedra was banned in 2004 due to its health risks, several similar stimulants took their place. Most advertise that they are ephedra-free and safe.

These often contain bitter orange (citrus aurantium), synephrine, or octopamine. Two of the most popular products, Xenadrine EFX and Advantra Z, were tested by researchers in 2005 and still found to have unsafe effects on heart rate and blood pressure.

Meridia

Meridia (sibutramine), an appetite suppressant, was removed from the market in the United States in 2010. The FDA initially approved the product, but the manufacturer stopped producing it after clinical studies showed that users had an increased risk of heart attack and stroke.

Fenfluramine

This drug was part of the anti-obesity medication fen-phen and worked by reducing carbohydrate cravings. However, the FDA withdrew its approval in 1997 after receiving numerous reports that women taking this drug had developed valvular heart disease.

Dexfenfluramine

This appetite suppressing drug—also known as Redux— was approved for a short period of time in the 1990s. Still, the FDA ultimately withdrew its approval after concerns about its effects on the cardiovascular system. Like fenfluramine, it appeared to cause valvular heart defects.

Belviq

Belviq (lorcaserin) worked by activating serotonin receptors that regulate hunger. It was available with a prescription to patients with a BMI of 30 or a body mass index of 27 along with an obesity-related condition. In 2020, however, the FDA recalled Belviq from the market, citing cancer risks.

The fact that these banned drugs were once approved by the FDA serves as a reminder that even with FDA approval, there can be unknown risks of taking weight-loss drugs.

Factors to Consider

There are a few factors to think about before deciding to buy and use diet pills. These include:

  • Your medical history: Taking weight loss pills could be dangerous for people with some conditions, such as heart disease, liver disease, hyperthyroidism, or glaucoma.
  • Other medications you take: Some diet pills may interact with other drugs, either intensifying or reducing their effects.
  • Allergies: If you are allergic to any substances that could potentially be included in the weight loss pill, it's important to avoid taking it.
  • If you are pregnant or breastfeeding: You could potentially pass the ingredients on to your newborn or unborn child.

A Word From Verywell

Diet and lifestyle changes are usually the best way to support lasting weight loss and maintenance. However, there are instances when health care providers may suggest weight loss supplements or prescription medications based on your individual needs.

Always talk to your doctor about any diet pill or weight loss supplement that you are considering. This helps ensure that it is safe for you, given your health and physical condition.

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