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Phentermine interactions to avoid

Hypertension, seizures, or serotonin syndrome are symptoms that may occur due to phentermine interactions
By | Dec. 11, 2023
Rx pill bottle and Rx prescription pad: Phentermine interactions

Key takeaways

  • When combined with phentermine, monoamine oxidase inhibitors (MAOIs) can cause deadly blood pressure spikes due to exacerbated hypertensive effects.

  • Combining antipsychotic medications or the antidepressant bupropion with phentermine increases seizure risk.

  • The interaction between selective serotonin-reuptake inhibitors (SSRIs) and phentermine can lead to serotonin syndrome. The combo can also increase phentermine’s stimulant effects, potentially causing more cardiovascular side effects.

  • Because phentermine has a stimulant effect that can lower blood pressure, it can counteract the effectiveness of antihypertensive drugs.

In this article:

Phentermine is an appetite suppressant approved by the Food and Drug Administration, FDA, for short-term use in weight management. You may know it as brand-name Adipex-P, or you could be aware of it being an ingredient in Qsymia (a combination of phentermine and topiramate). 

Concerns abound regarding the drug’s potential for serious side effects, namely cardiovascular or neurologic. These ill effects can be compounded by the concurrent use of other drugs that impact the heart, blood pressure, or nervous system. On the list of drugs that interact with phentermine, the headliners are antihypertensives, antipsychotics, and antidepressants, including bupropion, monoamine oxidase inhibitors, and selective serotonin-reuptake inhibitors. 

Even alcohol and caffeine can create problems when taking this stimulant. 

The potentially severe nature of these interactions means you should bring any concerns to your medical provider and quickly report any symptoms, such as headache, tremor, or confusion. Informing yourself about phentermine interactions is wise before or during a treatment course.

RELATED: Phentermine side effects

Phentermine-drug interactions

While phentermine has a lengthy list of drug interactions, most of them boil down to causing hypertension, seizures, or serotonin syndrome.

Monoamine oxidase inhibitors

MAOIs are the most notorious medicine class for dangerous food and drug interactions. For this reason, they are one of the last resort antidepressant options. In addition to antidepressants in the class, some MAOIs (Azilect and selegiline) are used for Parkinson’s disease, and Zyvox is an antibiotic. 

An adrenaline surge triggered by MAOIs can set off a hypertensive crisis. It makes sense that stimulant medications, including phentermine, could magnify adrenaline effects, sending blood pressure soaring. If you take an MAOI, you should be vigilant about avoiding drug interactions, including phentermine. 

Any MAOI would have to be stopped at least 14 days in advance of starting phentermine. 

Examples of MAOIs include:

Antipsychotics

Phentermine is considered a drug that decreases the seizure threshold, meaning that a seizure is more likely to occur, and antipsychotics have this characteristic too. Mixing phentermine with antipsychotics can be a recipe for disaster for seizures. 

Specific examples of the antipsychotics in question include:

Considering that antipsychotics are associated with a side effect of weight gain, it is not far-fetched to imagine a scenario in which concurrent phentermine would be considered for obesity management while taking an antipsychotic. If you find yourself in such a scenario, it is worth carefully reviewing the seizure risk with your healthcare provider. 

Bupropion

Bupropion is another medication from the mental health field that can increase the risk of seizures. Taking the antidepressant bupropion with phentermine compounds the danger. If you take generic bupropion or brand-name forms, like Wellbutrin SR and Wellbutrin XL, a seizure risk assessment and discussion of alternative weight management meds would be worthwhile before starting phentermine. If you have a seizure disorder, these meds pose an even greater threat.

Selective serotonin-reuptake inhibitors

The widely agreed-upon first-line medicines for depression come from the SSRI family. Members of this antidepressant class interact with phentermine in a couple of different ways. 

Phentermine can promote the serotonin effects of the antidepressant to such an extent that serotonin syndrome results, leading to life-threatening changes to body temperature regulation and the neurologic system. 

In addition, SSRIs can exacerbate the appetite suppressant’s stimulant effects, resulting in more cardiovascular side effects. 

Being aware of this bidirectional drug interaction is beneficial, but you should bring it up with your healthcare provider. You can ask whether other weight-loss medications may be a better option, such as one of the glucagon-like peptide 1 (GLP1) agonists like Wegovy or Saxenda. 

If you are unsure if your antidepressant is an SSRI, here are some examples:

Antihypertensives

A more intuitive interaction is the one between phentermine and antihypertensives. The appetite suppressant’s tendency to raise blood pressure is directly opposed to the antihypertensive treatment goal. This is true across all blood pressure medications, including the following examples:

If you deal with high blood pressure, looking for a different weight-loss aid might make sense. You can ask your healthcare provider about alternatives, such as a GLP1 agonist, Contrave (bupropion-naltrexone), or Plenity.

Phentermine-food interactions

Consuming lots of chocolate is probably not part of your calorie-cutting and weight-loss strategy. If you are taking phentermine, you have even more reason to avoid chocolate. Chocolate is a mild stimulant, and accordingly, it could interact with phentermine to increase seizure risk. Putting a limit on your chocolate treats is worthwhile for multiple reasons. 

Other phentermine interactions

The trouble with phentermine does not stop with food and drug interactions. In fact, a broad range of substances do not play well with phentermine.

Phentermine and alcohol

According to the Adipex P drug label from the Food and Drug Administration (FDA), side effects of phentermine can be heightened by alcohol. The specifics are not explained, but the prudent move is to avoid alcohol while taking phentermine.

RELATED: Learn more about phentermine and alcohol here

Phentermine and caffeine

Your morning cup of coffee or beloved afternoon tea could be troublemakers with phentermine. Caffeine is a stimulant, as is phentermine, and therein lies the problem. Dual stimulants could increase the likelihood of hypertension or seizures. You can talk with your healthcare provider about what limits may be appropriate.

RELATED: Learn more about phentermine and caffeine here

Phentermine and cocaine or methamphetamine

If coffee and chocolate are called out as stimulants to limit or avoid phentermine, you can bet that cocaine and methamphetamine must be strictly avoided in the setting of phentermine therapy. A dangerous rise in heart rate and blood pressure could occur otherwise.

While phentermine is technically termed a sympathetic amine anorectic drug, it is very similar to amphetamine. It has abuse potential, too, making it a hazard for those with a history of drug abuse.

Phentermine and cannabinoids

Putting together cannabinoids and phentermine could have your heart pounding. The combination can bring on tachycardia (a fast heart rate), so steering clear is wise.

Phentermine and cardiovascular disease

A history of congestive heart failure, heart disease, uncontrolled hypertension, arrhythmias (irregular heartbeat), and stroke could all be exacerbated by faster heart rates and higher blood pressure. Given that phentermine can produce tachycardia, hypertension, palpitations, and chest pain, it is typically avoided in any of these circumstances. These cardiovascular conditions are considered contraindications to its use.

Phentermine and hyperthyroidism

Hyperthyroidism results from an overactive thyroid gland producing an excess amount of our body’s own natural stimulant, thyroid hormone. The use of phentermine in the setting of hyperthyroidism could be dicey. Symptoms like tachycardia and tremor could be experienced. Your care team may suggest a different option to reduce your body mass index (BMI), a measure of weight compared to height. 

Phentermine and glaucoma

Stimulants like phentermine can cause your pupils to dilate, an action which may increase pressure in the eye. Since glaucoma treatment goals are to reduce and control intraocular pressure in the eye, phentermine can pose a problem. Get the medical advice of your eye care provider before starting phentermine if you have glaucoma.

Phentermine and renal impairment

If you have kidney disease, levels of phentermine could climb higher when taken. This exposure would put you at more risk of adverse effects. Avoiding phentermine altogether or adjusting the dose are options for you to review with the prescribing healthcare professional.

How to minimize phentermine interactions

Drug interactions can be avoided or at least minimized by staying organized and engaged with the details of your care. 

Step one is being conscientious about maintaining your med list. Keep an updated listing of your prescription drugs, over-the-counter medications, and supplements. 

Next, make sure your healthcare team is aware of everything on that list. 

Finally, ask them whether any drug-drug interactions are present and what actions can be taken to mitigate them.

When to talk to a healthcare provider about phentermine interactions

You are now better informed of the perils of phentermine’s drug interactions and some specific examples. Certainly, other interactions exist besides the ones detailed here. The most important message is to feel empowered to ask questions. As a general practice, when you are prescribed a new medicine, ask your healthcare provider about whether it interacts with anything on your med list. This dialogue can be a lifesaver.

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