Weight loss and diet pills, examined by scientists

Ever wonder what goes into a weight-loss pill, or whether they’re even safe? Read on to see what the science says about different weight loss pills and supplements.
Published March 12, 2020

While every person’s weight-loss journey is unique, there’s typically a common thread: the realization that losing weight takes effort and time. There’s the food prep, the gym sessions, and the dedication to ditching unhealthy habits and behaviours. So it’s no wonder that weight-loss pills appeal to those looking to drop pounds ASAP: In fact, about 15 percent of adults in the U.S. have tried a weight-loss supplement at some point in their lives, according to the National Institutes of Health.


So could popping a pill really be the slim-down shortcut we’ve been looking for? Not so fast. You’ve heard it before, but if something sounds too good to be true, it probably is, and weight-loss drugs are no exception. The pills (which include prescription medications, OTC drugs, and dietary supplements) aren’t intended to be a fast, easy weight-loss solution. “They can be a helpful piece of the puzzle, but they’re not a quick fix,” says Kent Sasse, M.D., a bariatric medicine specialist in Reno, Nevada and author of The Type II Diabetes Cure. In some instances, these pills could even put your health at risk. Read on to learn more about how weight-loss pills work.


How do weight loss pills work?

Weight loss pills work—or claim to work—in a number of ways depending on the product’s ingredients. Some rev up the metabolism to increase fat burn, while others curb your appetite or limit how much fat the body can absorb from foods, Dr. Sasse says.


Prescription vs. over-the-counter weight loss pills

Doctors sometimes recommend prescription weight-loss pills combined with diet and exercise to patients who have significant amounts of weight to lose—typically a body-mass index (BMI) of 27 or higher. But you don’t need an Rx to stock up: Many diet pills and supplements are sold over-the-counter. Of course, that doesn’t necessarily mean they’re safe for everyone to take. In fact, some of these products can have scary health consequences, such as liver damage or heart trouble. It’s why you should always get the green light from your physician before popping a new medication or supplement—even if it’s one that you buy OTC.


Are weight loss pills safe?


This depends. “The drugs that have been approved by the FDA have gone through extensive clinical trials and have a very low side-effect profile,” Dr. Sasse says. But as with all medications and supplements, diet pills can come with unpleasant or damaging side effects and risks—even if they’ve been tested in clinical trials and approved by the FDA.

Another warning: Pharmacy and supplement store shelves are lined with weight-loss pills that have limited—or zero—research behind them, so the long-term effects of taking the drugs can be unclear. Meanwhile, some options are plain unsafe. In fact, certain weight-loss pills that were initially approved by the FDA, such as dexfenfluramine (Redux) and fenfluramine (Fen-Phen), were only discovered to be associated with scary health risks (and had their FDA approvals withdrawn) after being released on the market.


The science behind 7 common weight loss pills


The diet pills market is crowded with products, but these seven weight-loss pills and supplements are some of the most common. Here, a look at how they work, what the science says about efficacy and safety, and information about potential side effects.


1. Orlistat (a.k.a. Xenical and Alli)


Orlistat is available in both prescription capsule form (Xenical) and as an over-the-counter capsule (Alli). Alli is approved for use in adults who have a BMI of 25 or higher while Xenical is intended for those who are overweight or obese and may also have diabetes, heart disease, or high blood pressure or cholesterol.

How it works: Orlistat decreases the amount of dietary fat that the intestines absorb; the fat that’s not soaked up is then eliminated from the body during bowel movements.

That said, Orlistat doesn’t give you license to eat everything in sight. To be effective, the pill needs to be used in conjunction with a reduced-calorie, low-fat diet.

Orlistat is typically taken three times per day with meals. It’s recommended that people on Orlistat also take a multivitamin to fill in nutritional gaps that may occur when the body doesn’t absorb certain vitamins.


What science says: In a six-month study published in the Journal of Obesity, participants who took Xenical not only lost weight and lowered their BMI but also started to eat more healthy foods like fruits and vegetables and curbed their intake of unhealthy eats like chips, cake, and fried foods. Another study found that, compared to taking a placebo, Orlistat led to a significant reduction in weight along with BMI, waist circumference, total cholesterol, and LDL (“bad” cholesterol) levels.

Side effects: Orlistat’s most common side effect is GI trouble. These symptoms may include:

  • Oily spotting on underwear after passing gas
  • Loose or oily stools
  • Diarrhea
  • Difficulty controlling bowling movements or the urgent need to go
  •  Abdominal pain

Additionally, taking Orlistat may cause the following:

  • Irregular menstrual cycles
  • Headaches
  •  Anxiety
  • Hives, itching, or rash
  •  Trouble breathing or swallowing
  • Fatigue or weakness
  • Loss of appetite
  • Nausea or vomiting

Orlistat’s side effects are more likely to be experienced if you eat a diet that’s high in fat, which is why it’s advised that people avoid foods that contain more than 30 percent fat, like steak, avocado, and certain processed and fast foods.


2. Hydroxycut


If you’ve spent any time shopping the supplement aisles, you may have come across Hydroxycut, a dietary supplement that’s seen formulation changes over the years. Hydroxycut originally contained ephedra, a plant-based weight-loss ingredient, but in 2004, the FDA banned the sale of products containing ephedra due to cardiovascular concerns. Now multiple Hydroxycut formulations exist. Depending on the product, they may contain ingredients such as caffeine and green coffee bean extract.

How it works: Hydroxycut claims that its products aid weight-loss by boosting metabolism. The range of the line includes capsules, gummies, and drink mixes, which are intended to be taken twice daily.

What science says: Hydroxycut has come under scrutiny due to a lack of evidence that the products live up to their claim. Moreover, Hydroxycut may pose a health risk—even now that ephedra has been cut from the formulation. It’s also been linked to acute liver injury, although it’s unclear which ingredient is to blame.

Side effects: The new Hydroxycut formulation comes with a number of side effects, which may include:

  • Difficulty sleeping or insomnia
  • Anxiety
  • Nausea
  • Headache
  • Increased heart rate and blood pressure
  • Diarrhea
  • Vomiting

3. Contrave


Available by prescription, Contrave is a combination of two generic drugs: bupropion, an antidepressant, and naltrexone, which is used for drug addiction.

How it works: Contrave acts on the brain to decrease appetite and reduce the desire to comfort eat. Those on the medication start by taking one tablet each morning and gradually work their way up to taking two tablets twice a day. The slow increase in dosage allows the body to adjust to the medication.

What science says: Contrave leads to greater weight loss than diet or exercise alone, according to a randomised, double-blind, placebo-controlled clinical trial. The same 56-week study showed that patients taking Contrave lost 5.4 percent of their body weight on average.

Side effects: Contrave’s side effects may include:

  • Nausea and vomiting
  • Headache
  • Constipation
  • Dizziness
  • Insomnia
  • Seizures
  • Diarrhea
  • Increased blood pressure and heart rate
  • Increased risk of suicidal thoughts
  • Seizures
  • Increased suicidal thoughts

Contrave shouldn’t be taken by those with a seizure disorder, uncontrolled high blood pressure, or a narcotic addiction (or who are on medication to treat such an addiction).


4. Belviq


When paired with diet and exercise, this prescription weight-loss medication may help obese or overweight adults lose weight.

How it works: Belviq activates receptors in the brain that boost feelings of satiety. In other words, you can feel full while eating less. The pill is available in two different doses. The 10mg tablet is taken twice daily. The 20mg tablet is taken once a day.

What science says: Research has shown that Belviq is effective for weight loss. One clinical trial found that, after two weeks of use, patients taking Belviq lost 3.3 pounds while those on a placebo lost 2 pounds.

Side effects: Belviq’s side effects may include:

  • Headache
  • Dizziness
  • Fatigue
  • Nausea
  • Dry mouth
  • Constipation

People with diabetes are at risk of additional side effects, including low blood sugar and back pain. If patients haven’t lost 5 percent of body weight by week 12 of treatment, they should stop taking Belviq. 


5. Saxenda


Saxenda, a brand name for the medication liraglutide, is a prescription weight-loss injection that’s self-administered once daily. When combined with a low-calorie diet and increased physical activity, it can help adults with obesity as well as those who have a BMI of 27 or higher and a weight-related medical condition lose weight and keep it off.

How it works: The body naturally produces a hormone (GLP-1) that regulates hunger. When injected into the stomach area, upper leg, or upper arm, Saxenda mimics this hormone, helping to control appetite and reduce eating.

What science says: About 62 percent of those on liraglutide lost at least 5 percent of their body weight (compared with 27 percent of those on a placebo), shows a randomized, controlled trial published in The New England Journal of Medicine.

Thirty-three percent of patients on liraglutide lost more than 10 percent of their body weight.

Side effects: Saxenda’s side effects may include:

  • Nausea
  • Diarrhea
  • Constipation
  • Headache
  • Vomiting
  • Low blood sugar
  • Upset stomach or stomach pain
  • Fatigue
  • Dizziness

Saxenda’s more serious but less common side effects include thyroid tumors (including cancer), gallbladder problems (including gallstones), kidney failure, and inflammation of the pancreas.


6. Qsymia


Available by prescription, Qsymia is a combination of two medications: phentermine, which reduces appetite, and topiramate, which is used to treat seizures and migraines. Qsymia is prescribed to help adults who are overweight or obese with weight-related medical problems lose weight—and keep it off.

How it works: Qsymia helps curb hunger and make people feel full sooner, allowing them to eat less. 

What science says: Two large trials conducted in 2012 found that, after one year of treatment, people on Qsymia lost significantly more weight than participants who took a placebo. 

Side effects: Qsymia’s side effects may include:

  • Constipation
  • Dizziness
  • Dry mouth
  • Tingling of the hands and feet
  • Trouble sleeping and excessive tiredness

Because Qsymia may lead to birth defects, women who are pregnant or planning to become pregnant should not take Qsymia. It should also be avoided when breastfeeding or if you have glaucoma or hyperthyroidism.


7. Raspberry ketones


Raspberry ketones are a compound found in raspberries that give them their aroma. Originally used in products such as ice creams and cosmetics to enhance flavour and taste, raspberry ketones are now marketed as a weight-loss-enhancing dietary supplement.

How it works: Manufacturers of raspberry ketone supplements claim that their products aid weight loss by breaking down fat cells and regulating metabolism.  

What science says: There’s only one randomized controlled trial, published in The Journal of the International Society of Sports Medicine, that examines raspberry ketones and weight loss. The study, which looked at 70 obese but otherwise healthy patients, found that those who took a multi-ingredient supplement containing raspberry ketone, caffeine, bitter orange, ginger, and other ingredients lost 2 percent body weight after eight weeks while those who took a placebo lost 0.5 percent. But because the weight-loss product contained multiple ingredients, it’s impossible to know whether the results are directly linked to raspberry ketones.

Side effects: More research is needed to understand the full range of side effects, but there are currently no known serious adverse reactions.


Should you take weight loss pills?


Whether you should take prescribed medications is up to you and your doctor. But when it comes to dietary supplements that are not overseen by the FDA, or other OTC diet pills, the side effects, financial cost, and unknown or potentially harmful impact on your long-term health is simply not worth the risk.

“The best case scenario from taking a diet pill for the purpose of weight loss—that is, losing a nominal amount of weight without developing any strategies to maintain the loss over time—is like asking a patient to put a bandaid on a gaping wound,” says registered dietitian Jaclyn London, the head of nutrition at WW. “The worst case scenario is side effects that make you feel physically sick, wasting money on a treatment that doesn’t work, or/and increasing your risk of potentially long-term health complications.”

What’s more, even an effective weight-loss pill won’t address the underlying psychological factors that can contribute to a person’s weight. “Medication is a quick fix to a problem that requires a deep-dive into what contributed to weight-gain in the first place—environment, psychosocial factors, level of physical activity, history of disordered eating patterns, food and weight history, food security, exposure to nutrition education, genetic factors, and much, much more,” London says, adding that only certain instances warrant weight-loss medications. “It’s worth an in-depth conversation with your physician.” 

That said, you and your doctor should also consider the side effects and possible drug interactions before taking diet pills—and remember there are certain people who should never take diet pills: If you’re pregnant or lactating; have a history of disordered eating patterns; or have diabetes or are taking medication or insulin for the treatment of hyperglycemia, it’s best to steer clear.

As for the rest of us, the bottom line: “There’s no such thing as a weight-loss pill or supplement that can work in isolation of everything else you do all day,” London says. “Even if it’s OTC, you should check with your physician before you start any new intervention.”

Not psyched about the prospect of potential health risks, stomach aches, nausea, and more? Dodge these reactions altogether by considering other weight-loss methods, such as reducing calorie intake, engaging in physical activity, and following a healthy eating pattern. 


Rachel Morris is an executive editor at WW. She was previously on staff at Dr. Oz The Good Life, Woman’s Day, and Parents magazines and has written for publications including O, The Oprah Magazine, Women’s Health, and Martha Stewart Living. Her favourite ZeroPoint snack: Mango! 


Reviewed by Jackie London, R.D., November 2019