Many dieters turn to natural supplements that help with weight loss, including calcium and green tea extract, but medication, whether it’s over the counter or prescription, can have a much stronger effect, especially if you’re struggling with obesity.

Find out more about the most popular diet pills that work, including their side-effects and other restrictions.

Orlistat (Alli, Xenical)

Available over the counter as Alli, or with a prescription as Xenical, Orlistat has been approved by the FDA thanks to studies that show it can block your body from absorbing over 25% of the fat you eat. Orlistat is approved for long-term use, but its side-effects may include gas, cramps, more frequent bowel movements and oily stool. 

Even if it’s one of the diet pills that work, Orlistat has the most beneficial effects when you’re on a low fat diet, and you also need to take a multivitamin every day, since it can can block the absorption of some vitamins. Unlike most diet pills, Orlistat doesn’t affect your appetite.

Sibutramine (Meridia)

A prescription-only weight-loss drug, sibutramine, available as Meridia, keeps your appetite in check by changing your body’s levels of serotonin and norepinephrine, which leads to satiety. Clinical trial with over 12,000 people have shown it can help dieters lose up to 10 pounds more in a year, but so far it’s only approved for obese and overweight people, with a BMI of 27 or higher. Side effects include raised blood pressure and a higher risk of stroke and heart attack. It’s definitely one of the diet pills that work, but you’ll benefit the most if the reason why dieting hasn’t helped you is the fact that you don’t feel full.

Lorcaserin (Belviq)

Another appetite-suppressant drug approved for long-tem use, lorcaserin is definitely effective in some people, but it doesn’t work for everyone. According to the FDA, if you take the drug for 3 months without losing 5% of your weight, it’s unlikely to work for you. It’s not an option for pregnant women or those who plan on getting pregnant, and it can also have serious interactions with depression medication. Even though it’s one of the diet pills that work for many, its side-effect can include headaches, nausea, fatigue, dizziness, constipation and dry mouth.

More: Diet Foods That Make You Gain Instead of Losing Weight  

Metformin (Glucophage) and Exenatide (Byetta)

Prescribed in combination to induce weight loss in obese diabetic patients, metformin and exanetide, have two different effects. Glucophage is used to increase insulin sensitivity and lower blood sugar levels, while the injectable Byetta keeps food in your stomach for longer, helping with satiety and decreasing hunger pangs. The two drugs are only prescribed to diabetics with a BMI of 25 or higher and side-effects include diarrhea, nausea and stomach cramps. The good news is that they’re among the diet pills that work, with very high chances of keeping off the weight after losing it.

Phentermine (Adipex, Suprenza)

An amphetamine that’s only prescribed for short-term use, phentermine, also know as Adipex or Suprenza, can reduce your appetite, but with many side effects and a significant risk of addiction. While diabetics need to adjust their insulin dose when taking the drug for a few weeks, side effects for everyone include everything from insomnia and tremors to heart palpitations, chest pain, and dizziness. It shouldn’t be taken in the evening, and it can also affect your ability to drive or operate machinery.

Phentermine / Topiramate (Qsymia)

Mixing phentermine with topiramate, a migraine and seizure drug, Qsymia can be used in the long term, because it contains a much smaller dose of the amphetamine, while adding important benefits for weight loss. From feeling full for longer, to burning calories faster, the combination of the two drugs can help you lose up to 3% of your body weight in 3 months. If that doesn’t happen, a doctor may decide to increase your dosage, but if the weight loss effects still haven’t arrived in 3 more months, the FDA recommends phasing out the drug. Side effect include an altered sense of taste (making food taste less appealing), dizziness, constipation, dry mouth and insomnia.

See also: Best Plant Based Diets for Weight Loss