Diet pills work through four main strategies: suppressing appetite, blocking fat absorption, increasing calorie burn, and reducing food cravings. The specific approach depends on whether you’re looking at FDA-approved prescription medications or over-the-counter supplements, and the gap in effectiveness between those two categories is significant. Here’s how each type works and what the evidence actually shows.
Appetite Suppression: The Most Common Approach
The majority of prescription weight loss medications work by making you feel less hungry or full sooner. They do this by targeting different systems in the brain, but the end result is the same: you eat less without feeling like you’re fighting constant hunger.
The oldest approach uses a stimulant called phentermine, which is still the most commonly prescribed weight loss medication in the United States. It triggers the release of a stress hormone in the part of the brain that controls hunger. That signal tells your brain you’re not hungry and slightly increases the number of calories your body burns at rest. Phentermine is only approved for short-term use, typically a few weeks, because its stimulant effects can become problematic over time.
The newer generation of appetite-suppressing drugs works differently. Medications like semaglutide (Wegovy) and liraglutide (Saxenda) mimic a gut hormone called GLP-1 that your body naturally produces after eating. This hormone signals your brain that you’ve had enough food. By keeping those signals active for longer, these drugs reduce appetite in a way that feels more natural, like your body simply isn’t asking for as much food. Tirzepatide (Zepbound) takes this a step further by mimicking two gut hormones simultaneously, GLP-1 and another called GIP, which appears to amplify the effect. These GLP-1 based medications are approved for long-term use, a major shift from older short-term options.
Blocking Fat Absorption in the Gut
One medication takes a completely different route: instead of changing how much you want to eat, it changes how much of what you eat your body actually absorbs. Orlistat (sold as Xenical by prescription or Alli over the counter) works in your digestive tract by permanently disabling the enzymes that break down dietary fat. When those enzymes can’t do their job, fat passes through your system undigested rather than being absorbed into your bloodstream.
The mechanism is straightforward. Your body relies on specific enzymes in the stomach and intestines to split fat molecules into pieces small enough to absorb. Orlistat bonds irreversibly to these enzymes, shutting them down. The undigested fat simply exits the body. This is also what causes the drug’s most notorious side effects: oily stools, gas, and urgent bowel movements, especially after high-fat meals. Many people find that the side effects themselves serve as a behavioral motivator to eat less fat.
Targeting Food Cravings and Reward Signals
Some weight gain is driven less by physical hunger and more by cravings, the pull toward food for comfort or pleasure rather than fuel. One prescription combination, naltrexone-bupropion (Contrave), was designed specifically to address this. It pairs two existing medications: one used to treat addiction and another used to treat depression and help people quit smoking. Together, they act on the brain’s reward circuitry and the hunger-regulating system in the hypothalamus.
The idea is that for some people, overeating has a compulsive quality similar to other reward-driven behaviors. By dampening the pleasure signal food provides while simultaneously reducing appetite, the combination addresses both the “I’m hungry” and the “I want that” sides of overeating. This medication does carry a risk of increased blood pressure and palpitations, so it’s not appropriate for everyone.
Side Effects Vary by Drug Type
Each mechanism comes with its own set of trade-offs. The GLP-1 based drugs (semaglutide, liraglutide, and tirzepatide) are most commonly associated with gastrointestinal issues: nausea, vomiting, and diarrhea, particularly during the first weeks as the dose is gradually increased. These effects tend to lessen over time for most people.
Phentermine-topiramate, which combines a stimulant with a seizure medication to amplify appetite suppression, carries risks of anxiety, sleep problems, and irritability. The topiramate component has also been linked to increased risk of depression. Naltrexone-bupropion can raise blood pressure. And orlistat’s digestive side effects, while not dangerous, are unpleasant enough that many people stop taking it.
Who Qualifies for Prescription Options
Prescription weight loss medications aren’t available to anyone who wants to lose a few pounds. The FDA approves them for people with a BMI of 30 or higher (classified as obesity) or a BMI of 27 or higher when combined with at least one weight-related health condition such as type 2 diabetes, high blood pressure, or high cholesterol. Six medications are currently approved for long-term use: orlistat, phentermine-topiramate, naltrexone-bupropion, liraglutide, semaglutide, and tirzepatide. Short-term options like phentermine alone are approved for just a few weeks.
This distinction matters because weight management is increasingly treated as a chronic condition. The newer long-term medications represent a shift away from the old model of taking a pill for a few weeks and hoping the results stick. Most people who stop these medications regain weight, which is why they’re designed for ongoing use alongside diet and exercise changes.
Over-the-Counter Supplements: Weak Evidence
The supplement aisle tells a different story. Dozens of products claim to boost metabolism or burn fat, but the evidence behind most of them is thin.
Green tea extract is one of the most studied. It’s proposed to increase energy expenditure and fat burning. A Cochrane Review of 14 trials involving over 1,500 people found that green tea supplements reduced body weight by an average of just 0.95 kg (about 2 pounds) more than a placebo. When researchers looked only at studies conducted outside Japan, the effect disappeared entirely. A separate analysis found that green tea combined with caffeine reduced weight by about 1.38 kg (3 pounds) and waist circumference by about 1.93 cm over 12 weeks, but green tea without caffeine had no measurable effect. The European Food Safety Authority concluded that no cause-and-effect relationship exists between green tea and weight management. There are also increasing reports of liver damage from concentrated green tea extract supplements.
Conjugated linoleic acid (CLA) is another popular supplement. While it reduces body fat in animal studies, human results are modest at best. One 12-week trial found it reduced body weight by just 0.69 kg (about 1.5 pounds) compared to a placebo. A larger trial showed reductions in body fat mass of 7 to 9 percent, but even those results are small in absolute terms.
The gap between these supplements and prescription medications is substantial. Where over-the-counter products might produce a pound or two of additional weight loss over several months, prescription GLP-1 drugs in clinical trials have produced weight loss of 15 percent or more of total body weight. That’s the difference between a barely noticeable change and a transformation visible in how your clothes fit and how your lab work reads.