Taking topiramate and phentermine together may be more effective for weight loss compared to taking the drugs on their own.

Phentermine is an appetite suppressant that can help with weight loss and weight management in combination with diet and exercise.

Topiramate is approved for treating migraine and epilepsy, but it can be used off-label for weight loss. This means the Food and Drug Administration (FDA) has not approved topiramate by itself for this purpose.

This article looks at whether it’s best to take topiramate and phentermine separately or together for weight loss.

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Topiramate is a medication that’s used for migraine and epilepsy. According to a 2023 literature review, an off-label use for topiramate is weight loss, as it may help reduce calorie intake, reduce fat gain, and lower cholesterol and triglyceride levels.

Phentermine is an appetite suppressant, which means it may work by itself as a short-term treatment for weight loss and weight management alongside exercise and reducing calorie intake. It may help people lose weight in other ways, such as effects on your brain or your metabolism. It’s only approved to be used by itself for a few weeks.

Also, the FDA has approved a combination medication containing phentermine and topiramate to help with weight management. This combination is available in the brand-name drug Qsymia.

It’s important to note that phentermine and Qsymia are Schedule IV controlled substances. This means the drugs have approved medical uses, but they also carry a risk for misuse. Higher drug schedules carry a higher risk for misuse. For example, Schedule II drugs have a greater risk for misuse than Schedule III drugs.

Study results

A 2013 study looked at how much weight people may lose taking topiramate and phentermine together compared to separately. The study compared the effects of topiramate only, phentermine only, and combination topiramate/phentermine for weight loss.

The study included 756 participants. Researchers found that a combination of topiramate and phentermine resulted in greater weight loss compared to either topiramate or phentermine on their own.

After 28 weeks, the results showed patients in each of the treatment groups lost on average:

  • 20 pounds (lb) with combination treatment of 15 milligrams (mg) phentermine with 92 mg topiramate
  • 13.6 lb with 15 mg phentermine only
  • 14.7 lb for 92 mg topiramate only

This suggests taking the drugs together rather than separately may result in greater weight loss.

Topiramate is available as an oral tablet, oral capsule, and extended-release (ER) oral capsule. Phentermine is available as an oral tablet and oral capsule. These medications are taken by mouth.

Topiramate is typically taken once or twice per day, in the morning and at night with or without food. Phentermine is typically taken once per day in the morning, either before breakfast or 1 to 2 hours after breakfast.

You can take topiramate and phentermine at the same time. These two drugs are available in a combination brand-name drug called Qsymia, which is also typically taken once per day.

If you have questions about how to take these medications, talk with your doctor or pharmacist.

Below are answers to commonly asked questions about topiramate and phentermine.

Are there certain side effects that I should expect taking topiramate and phentermine together?

Side effects of taking topiramate and phentermine together may be similar to the side effects seen with each drug by itself.

In studies of phentermine, some of the side effects reported included high blood pressure, dry mouth, constipation, and changes in libido (sex drive).

Topiramate is not FDA approved for weight loss, so the side effects reported in studies of the drug are for when it’s being used for the drug’s approved uses, migraine and epilepsy. But side effects of topiramate when it’s used off-label for weight loss may be similar. Some of the more common side effects reported include tingling sensation, dizziness, fatigue (low energy), and nausea.

Phentermine and topiramate are also available as a combination drug under the brand-name Qsymia. In adults taking Qsymia, some of the more common side effects reported include:

  • tingling sensation
  • dizziness
  • dry mouth
  • insomnia
  • constipation

Some of the more common side effects reported in children ages 12 years and older taking Qsymia include depression, fever, and flu.

If you have questions about what side effects to expect with taking phentermine and topiramate, talk with your doctor or pharmacist.

What do I do if I am not losing weight on phentermine and topiramate?

If you are not losing weight with phentermine and topiramate, you should talk with your doctor. Your doctor may increase your dose of phentermine and topiramate to see if that helps with weight loss. They may also decide to stop treatment with these drugs and try a different option for you.

It’s important to slowly stop taking phentermine and topiramate, as suddenly stopping the drug may cause side effects. Be sure to follow any instructions from your doctor.

If you have questions about your weight loss while taking these drugs, talk with your doctor.

Taking phentermine and topiramate together may be more effective for weight loss than taking each drug on its own.

The FDA has approved a combination brand-name drug, Qsymia, that contains phentermine and topiramate. It can be used to help with weight management in combination with diet and exercise.

If you have questions about taking phentermine and topiramate together for weight loss, talk with your doctor or pharmacist.