Key Takeaways:
Phentermine (Adipex-P, Lomaira) is FDA approved for weight loss. You should avoid taking phentermine with monoamine oxidase inhibitors (MAOIs) and fenfluramine (Fintepla).
Phentermine can increase the risk of serotonin syndrome if you take it with antidepressants. It can also interact with alcohol, stimulants, and caffeine.
It’s best to avoid combining phentermine with other weight-loss medications unless your prescriber recommends it. If you’re taking medication for high blood pressure, talk to your prescriber about whether phentermine is a safe option for you.
When it comes to weight-loss medications, injectable options such as semaglutide (Wegovy) and tirzepatide (Zepbound) are often the first options that come to mind. But there are other options to help you lose weight that don’t require an injection and are often more cost effective. Phentermine (Adipex-P, Lomaira) is one example. It’s FDA approved for weight loss when combined with diet and exercise.
Though phentermine can help people lose weight, it has some risks to consider. This includes drug interactions, a few of which we’ll review below. This isn’t a complete list of interactions, but it’s a good place to start. Your healthcare team can check your medication list for any other phentermine interactions not listed here.
1. Stimulants
Phentermine is a stimulant medication. This means it can speed up the body’s functions, which is part of the reason it helps you lose weight. But phentermine’s stimulant effects can also speed up your heart rate and raise your blood pressure. So if you combine it with other stimulant medications, it could raise the risk of these side effects. The combination could also cause more serious side effects, such as tremors and an irregular heart rate.
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Stimulant medications include many attention deficit-hyperactivity disorder (ADHD) medications, such as amphetamine salt combo (Adderall). Pseudoephedrine (Sudafed), an over-the-counter decongestant, also has stimulant effects. It’s best to avoid taking phentermine with other stimulant medications.
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2. Caffeine and nicotine
Substances such as nicotine and caffeine are stimulants, too. So they can also make you feel jittery and increase your risk of heart-related side effects if you combine them with phentermine.
Talk to your prescriber if nicotine or caffeine are part of your daily routine. They may recommend a different weight-loss medication for you. Or they may want you to limit how much of these substances you consume while you’re taking phentermine.
3. Antidepressants
Serotonin is a chemical in the body that helps regulate many bodily functions. This includes helping you sleep, stabilizing your mood, and regulating bowel movements — just to name a few. Antidepressant medications are thought to treat depression by raising serotonin levels.
But if you take multiple medications that raise serotonin levels, it can be dangerous. When serotonin levels get too high, it can cause a serious medical condition called serotonin syndrome. The symptoms of serotonin syndrome can range from mild to severe. Mild symptoms include a fast heartbeat, sweating, and tremors. Severe symptoms include high fever, agitation, and dangerously high blood pressure.
Phentermine’s effects on serotonin are unclear. Some research says it can raise serotonin levels. Others say it doesn’t have a major impact on serotonin. But the manufacturer recommends avoiding phentermine within 14 days of taking a monoamine oxidase inhibitor (MAOI) due to the risk of a hypertensive crisis. MAOIs are a type of antidepressant known for causing drug interactions.
MAOIs are the only antidepressant that should never be combined with phentermine. Other antidepressants may be taken with phentermine as long as your prescriber OKs it. These include:
Selective serotonin reuptake inhibitors (SSRIs) such as sertraline (Zoloft)
Serotonin and norepinephrine reuptake inhibitors (SNRIs) such as venlafaxine (Effexor XR)
Tricyclic antidepressants (TCAs) such as amitriptyline
St. John’s wort (an herbal supplement)
If you’re taking one of these medications, talk to your prescriber before taking phentermine. They may ask you to look out for symptoms of serotonin syndrome. Or they may recommend a different weight-loss medication.
4. Fenfluramine
Fenfluramine (Fintepla) is a medication that treats seizures in people with Dravet syndrome or Lennox-Gastaut syndrome. Fenfluramine used to be prescribed with phentermine in an off-label combination for weight loss. It was referred to as “fen-phen.”
But these two medications were dangerous when combined — they caused damage to people’s heart valves. In 1997, the manufacturers of fenfluramine (and its “sister” medication dexfenfluramine) removed them from the market. It wasn’t until 2020 that fenfluramine was approved under the brand name Fintepla for Dravet Syndrome. In 2022 it was approved for Lennox-Gastaut syndrome.
Fenfluramine is most often used in children. And phentermine is only approved for adults. So it’s uncommon for someone to need phentermine and fenfluramine at the same time. But talk to your prescriber if it's been recommended that you take both these medications. They can help you decide what to do next, since they shouldn’t be combined.
5. Alcohol
It’s best to avoid drinking alcohol while taking phentermine. The combination may worsen phentermine side effects, such as dizziness, headache, and trouble sleeping. Phentermine can also make you more sensitive to alcohol’s effects and mask the symptoms of intoxication. So you may not be able to tell how impaired you are while drinking. What’s more, alcohol can also interfere with your weight-loss goals.
Talk with your prescriber if you have questions about alcohol and phentermine. They can help determine whether there’s a safe amount for you to drink. And they can help provide resources if you’re unable to stop or cut back on your drinking on your own.
If you or someone you know struggles with substance use, help is available. Call SAMHSA’s National Helpline at 1-800-662-4357 to learn about resources in your area.
Can you take phentermine with other weight loss medications?
In most cases, it’s best to take one weight-loss medication at a time. Combining them can raise your risk of side effects. And the safety of multiple weight-loss medications at once hasn’t been well-studied.
You shouldn’t combine phentermine with Qsymia. Qsymia already contains phentermine as an active ingredient along with topiramate ER, so taking more could raise your risk of side effects. It’s also best to use caution when combining phentermine with Contrave. Contrave contains a combination of bupropion and naltrexone. Bupropion is an antidepressant, so it can increase the risk of serotonin syndrome if combined with phentermine.
That being said, your prescriber may decide to combine phentermine with another weight-loss medication, such as semaglutide, in some cases. But you shouldn’t do this on your own without a prescriber’s OK.
Can you take phentermine with high blood pressure medications?
Yes, if your prescriber OKs it. High blood pressure is listed as a side effect of phentermine. But it’s unclear whether this is truly a concern. In fact, some studies have shown that phentermine actually lowers blood pressure, since it causes weight loss.
If you’re taking medication for high blood pressure, your prescriber will weigh the pros and cons of phentermine based on your medical history. They may have you keep a closer eye on your home blood pressure readings while you’re taking phentermine. Or they may decide a different weight-loss medication is a safer option.
Good to know: Phentermine isn’t recommended if you have a history of heart disease. But Wegovy, a newer weight-loss medication, may lower your risk of heart problems in addition to helping you lose weight. So it’s likely a better option than phentermine if you have a history of heart problems.
The bottom line
Phentermine (Adipex-P, Lomaira), is an oral weight-loss medication. You shouldn’t take phentermine with monoamine oxidase inhibitors (MAOIs) or fenfluramine (Fintepla). It’s also best to avoid combining stimulant medications, alcohol, and large amounts of caffeine with phentermine.
Taking phentermine with antidepressant medications can raise your risk of serotonin syndrome. So use caution when combining these medications. And you should only take phentermine along with another weight-loss medication if your prescriber recommends it. If you’re taking medication for high blood pressure, talk to your prescriber about whether phentermine is a safe option for you.
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References
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UCB, Inc. (2022). Fintepla (fenfluramine) oral solution now FDA approved for treatment of seizures associated with lennox-gastaut syndrome (LGS). PR Newswire.
U.S. Food and Drug Administration. (2020). FDA approves new therapy for dravet syndrome.
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Wong, G., et al. (2022). Combination anti-obesity medications to effectively treat bariatric surgery weight regain at an academic obesity center. Obesity Science & Practice.
