Diet pills work through a handful of core strategies: suppressing your appetite, slowing digestion so you feel full longer, changing how your body absorbs fat, or nudging your metabolism to burn slightly more calories. Some target a single pathway, while others combine two or three. The specifics depend entirely on whether you’re taking a prescription medication or an over-the-counter supplement, and the gap between the two in terms of proven effectiveness is enormous.
Appetite Suppressants and the Brain
The oldest category of diet pill works by changing the chemical signals in your brain that drive hunger. Phentermine, one of the most widely prescribed weight loss drugs, triggers the release of norepinephrine in the hypothalamus, the part of your brain that regulates appetite. Norepinephrine is a stress hormone, and when its levels rise in this region, your brain essentially dials down the hunger signal. To a lesser extent, phentermine also boosts dopamine and serotonin, both of which influence how rewarding food feels. The net effect is that you simply don’t think about food as much, and when you do eat, smaller portions feel satisfying.
Because phentermine is a stimulant, it comes with stimulant side effects: elevated blood pressure, insomnia, rapid heart rate, headaches, and a jittery or overstimulated feeling. These are the same trade-offs you’d expect from anything that revs up your sympathetic nervous system.
GLP-1 Drugs: Slowing Digestion, Resetting Appetite
The newer generation of weight loss drugs, the ones generating the most headlines, are GLP-1 receptor agonists. Semaglutide (sold as Wegovy) and liraglutide (Saxenda) mimic a gut hormone called GLP-1 that your body naturally releases after eating. Tirzepatide (Zepbound) takes this a step further by mimicking two hormones, GLP-1 and GIP, simultaneously.
These drugs work on two fronts. First, they slow gastric emptying, meaning food stays in your stomach longer. Your stomach relaxes, contracts less forcefully, and tightens the valve at its exit. The result is a persistent, physical sense of fullness that lasts well beyond a normal meal. Second, and potentially more important, they appear to interact directly with neurons in the hypothalamus that regulate your body’s “set point” for weight. Rather than just dulling hunger temporarily, they seem to recalibrate how much your brain thinks you should be eating.
The clinical results reflect this dual mechanism. In the SURMOUNT-1 trial, people taking the highest dose of tirzepatide lost 17.8% more body weight than those on placebo over 72 weeks. A follow-up trial showed that patients who had already lost at least 5% of their weight through diet and exercise alone went on to lose an additional 18.4% with tirzepatide, compared to the placebo group, which actually regained weight. These are dramatic numbers by the standards of weight loss medicine.
The downside is gastrointestinal. About 44% of people taking semaglutide experience nausea, 30% get diarrhea, and 24% deal with vomiting. These side effects are the most common reason people quit. More serious but rarer risks include pancreatitis, gallbladder problems, and gastroparesis (a condition where the stomach empties too slowly on its own). Tirzepatide carries a manufacturer warning about a potential link to thyroid tumors, though this has primarily been observed in animal studies.
Targeting the Brain’s Reward System
One prescription combination takes a completely different approach. Bupropion/naltrexone (Contrave) targets the reward circuitry that makes high-calorie food feel pleasurable. Bupropion, originally developed as an antidepressant, boosts dopamine and norepinephrine levels, which stimulates a group of brain cells that produce a compound called alpha-MSH. Alpha-MSH acts on specific receptors to reduce food intake and increase energy expenditure. On its own, though, bupropion’s effect is limited because the same brain cells also produce beta-endorphin, which circles back and shuts down the very pathway bupropion activated.
That’s where naltrexone comes in. Naltrexone blocks opioid receptors, preventing beta-endorphin from hitting the brakes. With that feedback loop disabled, bupropion’s appetite-suppressing signal runs stronger and longer. The combination also appears to influence the mesolimbic reward pathway, reducing the pull of palatable, calorie-dense foods specifically. Animal research has shown that this combination increases the density of certain dopamine receptors in reward-related brain regions, which may help explain why cravings for rich foods diminish. Because it can raise blood pressure, this combination is generally not suitable for people with uncontrolled hypertension.
Fat Blockers: Changing Absorption
Orlistat (available by prescription as Xenical or over the counter as Alli) works in the gut rather than the brain. It blocks enzymes that break down dietary fat, so roughly a third of the fat you eat passes through your body undigested and unabsorbed. This directly reduces the calories you take in from any given meal.
The mechanism is straightforward, but the experience can be unpleasant. Because undigested fat has to go somewhere, side effects include oily stools, gas, and urgent bowel movements, especially after high-fat meals. Many people find that these side effects act as a built-in behavioral motivator to eat less fat. Orlistat produces more modest weight loss than the newer drugs, but it remains the only weight loss medication available without a prescription in most countries.
How Over-the-Counter Supplements Work
The supplements lining pharmacy shelves use different, generally weaker mechanisms than prescription drugs. Most fall into two categories: thermogenics that aim to boost metabolism, and fiber-based products that try to fill your stomach.
Caffeine is the backbone of most thermogenic supplements. It works by slowing the breakdown of a molecule called cyclic AMP inside your cells, which keeps your metabolic rate slightly elevated. Green tea extract takes a complementary approach. Its active compounds, called catechins, slow the degradation of norepinephrine, letting it linger longer and sustain a mild metabolic boost. When caffeine and green tea catechins are combined, they act on two different steps of the same pathway, which is why so many supplements pair them together. Capsaicin, the compound that makes chili peppers hot, triggers a similar effect by stimulating the release of stress hormones from the adrenal glands, which temporarily raises calorie burning.
The word “mild” is doing heavy lifting in those descriptions. The metabolic boost from these compounds is real but small, typically amounting to a few dozen extra calories burned per day. That can add up over months, but it won’t produce dramatic results on its own.
Fiber-based supplements like glucomannan work mechanically. Glucomannan can absorb up to 100 times its weight in water, forming a thick gel in your stomach that increases the bulk of its contents. This gel triggers stretch receptors in the stomach wall, sending fullness signals to your brain through the vagus nerve and delaying gastric emptying. It’s essentially a way to trick your stomach into thinking you’ve eaten more than you have. The effect is modest and depends heavily on drinking enough water with the supplement.
Prescription vs. OTC: The Efficacy Gap
The difference in results between prescription medications and over-the-counter supplements is stark. Prescription combinations like phentermine/topiramate have produced sustained weight loss of up to 13% of body weight in clinical studies, with one trial showing an average loss of about 11% in patients who stayed on therapy for at least two years. Tirzepatide and semaglutide push those numbers even higher. No over-the-counter supplement has come close to these results in rigorous trials.
This gap exists because prescription drugs target powerful hormonal and neurological pathways with precision, while OTC supplements nudge metabolism or stomach volume in ways that are easy for the body to compensate for. Your body is remarkably good at defending its current weight. When you burn a few extra calories from caffeine, you may unconsciously eat a bit more or move a bit less. Prescription drugs succeed partly because they override these compensatory mechanisms at a deeper biological level, particularly the newer drugs that appear to reset the brain’s weight set point rather than just suppressing hunger temporarily.
Side Effects by Category
Side effects tend to cluster by mechanism. Stimulant-based pills like phentermine cause the problems you’d expect from a stimulant: insomnia, elevated heart rate, high blood pressure, headaches, and tremor. When phentermine is combined with topiramate, you may also notice tingling sensations, dizziness, altered taste, constipation, and dry mouth.
GLP-1 drugs are dominated by digestive issues. Nausea, diarrhea, vomiting, constipation, and abdominal pain are common, particularly during the first weeks as dosing gradually increases. Serious but less common risks include pancreatitis, kidney problems, and bowel obstruction.
Fat blockers cause gastrointestinal effects that are directly proportional to how much fat you eat. Fiber supplements are generally well-tolerated but can cause bloating, gas, or even bowel obstruction if taken without adequate water.
OTC stimulant supplements carry the same general risks as caffeine in high doses: jitteriness, anxiety, insomnia, and elevated heart rate. Because supplements are not regulated as strictly as prescription drugs, the actual dosage and purity can vary between brands, making side effects less predictable.