How do weight loss pills work?

Mechanisms of action

While all weight loss pills have one main goal – to help you lose weight – there are different kinds of pills that function differently.

Here is a look at some of the different types of weight loss pills and the specific ways in which they can help you shed pounds.

» LEARN: Do weight loss pills work?

Appetite suppressants

There’s an array of drugs, both prescription and over-the-counter, which aim to help you lose weight by curbing your appetite. Featuring ingredients like glucomannan and fenugreek, these medications target the brain’s hunger signals.

“Some drugs, like phentermine, stimulate the central nervous system to reduce cravings, while others, like GLP-1 medications (such as Wegovy and Saxenda), mimic hormones that tell your body it is full,” Lyons said. And when you’re able to feel full on less food, it’s easier to cut calories.

» MORE: Is GLP-1 safe?

Metabolism boosters

These weight loss pills boost your metabolism, which can help your body burn calories even when you’re not exercising. As you age, your metabolism naturally slows, which can make it easier to put on pounds, especially in midlife.

“Metabolism boosters contain stimulants like caffeine or green tea extract, which can slightly increase your heart rate and body temperature,” said Lyons. “While they might give you a little extra calorie burn, these types of weight loss pills tend to work best with a good diet and regular movement.” Metabolism pills can be found online or at vitamin and health food stores.

Fat blockers

Fat blockers prevent your body from absorbing some of the fat in your food. Alli is the only fat blocker sold over the counter; others, like Xenical, are prescription drugs. If you’re considering taking fat blockers, it’s a good idea to discuss your plans with your physician before getting started.

By lessening absorption, fat blockers allow a portion of dietary fat to pass through your body undigested, Lyons told us. “This can help with weight loss, but it can also cause digestive issues like bloating, gas and oily stools — especially if you eat a high-fat meal.”

Effectiveness of weight loss pills

There are a number of factors that determine whether weight loss pills will be effective for you. According to Lyons, “People who make lifestyle changes and take medication usually see the best results.” The following things can impact the effectiveness of weight loss pills:

• Genetics
• Diet
• Lifestyle
• Medical conditions
• Insulin resistance
• Hormonal imbalances

When combined with sustained lifestyle changes consistent with better health, like low-impact exercise and low-fat, healthier food choices, weight loss pills can be an effective part of your weight loss regimen. This is supported by clinical trials.

“Before a weight loss medication gets Food and Drug Administration (FDA) approval, it must undergo clinical trials to prove it works,” said Lyons. These studies compare people taking the medication with those taking a placebo, or a pill without the active ingredient.

On average, FDA-approved weight loss drugs help people lose about 5% to 10% of their body weight over several months. According to Lyons, “Medications like Wegovy (semaglutide) have shown even higher weight loss percentages in some trials.” But healthier lifestyle choices are necessary in order to keep the weight off.

Safety and side effects of weight loss pills

Weight loss pills can cause side effects, so it’s important to weigh the risks of medication against the possible risks of staying at a higher weight.

Many people live with obesity for years, and these medications can be a helpful option for weight management or personal health goals. If you’re considering them, talk to your doctor to ensure they’re safe and effective and that you understand any long-term risks.

» MORE: Are weight loss pills safe?

Common safety concerns

Some medications, like phentermine, are meant for short-term use because they can affect heart rate and blood pressure. Others can be used safely for longer periods of time to support your weight loss goals.

Here’s a list of weight loss medications that have been approved by the FDA for long-term use:

• Bupropion
• Liraglutide
• Naltrexone
• Orlistat
• Topiramate

When taking a long-term weight loss medication, you and your doctor should work together to examine any potential risks and side effects. These should be compared against the risks of continued obesity, like heart attack, stroke, diabetes and other conditions.

Common side effects

Side effects vary depending on the medication, Lyons told us. “Appetite suppressants may cause dry mouth, insomnia or jitteriness. Fat blockers can lead to stomach discomfort, oily stools or frequent trips to the bathroom. GLP-1 medications may cause nausea, vomiting or constipation, but these symptoms often improve as your body adjusts to the drug.”

Choosing the right weight loss pill

The right weight loss pill depends on your health, goals and lifestyle. An appetite suppressant might be a good fit if you struggle with cravings. A fat blocker like orlistat could help if you eat lots of fatty foods. A metabolism booster might help you with the metabolic slowdown that occurs naturally in middle age.

“If you opt for an over-the-counter supplement, ensure it contains clinically tested ingredients, has transparent labeling and is backed by research,” Lyons said. “Be wary of anything promising rapid results or a long list of unrecognizable ingredients. Supplements with high doses of caffeine or stimulants can also be risky, especially for people with heart conditions.”

» LEARN: How to get weight loss medications online

Here’s some other advice Lyons shares:

• Look for brands with transparent ingredient lists.
• Find drugs that have been thoroughly tested by a third party in clinical trials.
• Ensure the brand you choose is FDA-approved.
• Avoid anything that relies on celebrity endorsements, unrealistic promises or “miracle” claims.
• Check trusted medical sources for information about the drug you choose.
• Read reviews from actual users.
• Talk to your doctor.

Are weight loss pills worth it?

Yes, if you have personal weight loss goals, it may be worth speaking with your doctor about incorporating medications that can help support your efforts. FDA-approved weight loss pills have been clinically proven to help people lose weight effectively, but it’s important to keep in mind that they have the best results when paired with long-term lifestyle changes, like diet and exercise.

Be sure to ask your doctor which weight loss aids are safest and which will work best for you.