Understanding Phentermine
Phentermine is a prescription stimulant, similar to an amphetamine, that acts as an appetite suppressant [1.6.5, 1.8.4]. It works on the central nervous system to increase heart rate and blood pressure, while decreasing your appetite [1.6.4]. It is typically prescribed for short-term use (a few weeks) for individuals with a high body mass index (BMI) [1.2.3, 1.3.3]. Due to its mechanism of action and potential side effects—such as insomnia, dry mouth, dizziness, and increased blood pressure—it is critical to be aware of potential interactions with other drugs and substances [1.3.3, 1.8.4].
Absolute Contraindications: Monoamine Oxidase Inhibitors (MAOIs)
The most critical and dangerous interaction is between phentermine and a class of antidepressants known as Monoamine Oxidase Inhibitors (MAOIs) [1.4.2]. Combining phentermine with an MAOI can lead to a hypertensive crisis, which is a dangerously sharp increase in blood pressure that can be fatal [1.4.1, 1.4.5].
Because of this severe risk, phentermine is strictly contraindicated for use with MAOIs or within 14 days of discontinuing an MAOI [1.4.3, 1.9.4]. This 14-day washout period is essential to allow the body to clear the MAOI and prevent this life-threatening reaction [1.9.1, 1.9.3].
Examples of MAOIs include [1.2.2, 1.4.1]:
- Isocarboxazid (Marplan)
- Phenelzine (Nardil)
- Selegiline (Emsam, Eldepryl, Zelapar)
- Tranylcypromine (Parnate)
- Linezolid (Zyvox)
- Methylene Blue
Significant Drug Interactions
Beyond MAOIs, several other classes of medications can have significant interactions with phentermine, requiring caution and close medical supervision.
Antidepressants (SSRIs, SNRIs, and TCAs)
While MAOIs are an absolute contraindication, other antidepressants like Selective Serotonin Reuptake Inhibitors (SSRIs) and Serotonin and Norepinephrine Reuptake Inhibitors (SNRIs) also pose risks [1.5.3]. Combining these medications with phentermine can increase the risk of a rare but serious condition called serotonin syndrome [1.5.1, 1.5.3]. This condition occurs when there is too much serotonin in the body, leading to symptoms like agitation, confusion, rapid heart rate, high blood pressure, tremors, and high fever [1.5.3, 1.5.5]. The FDA label does not recommend co-administration of phentermine and serotonergic agents like SSRIs [1.5.2].
Examples of these antidepressants include [1.2.3]:
- SSRIs: Sertraline (Zoloft), Fluoxetine (Prozac), Paroxetine (Paxil)
- SNRIs: Venlafaxine (Effexor XR), Duloxetine (Cymbalta)
- TCAs: Amitriptyline
Other Stimulants and ADHD Medications
Since phentermine is itself a stimulant, combining it with other stimulant drugs can amplify its effects and side effects [1.3.2]. This includes other prescription stimulants used for ADHD, as well as over-the-counter decongestants. The combination can greatly increase the risk of elevated blood pressure, rapid heart rate, anxiety, insomnia, and in severe cases, heart attack or stroke [1.2.3, 1.3.2].
Examples of interacting stimulants include [1.2.3, 1.8.5]:
- Amphetamine/dextroamphetamine (Adderall)
- Lisdexamfetamine (Vyvanse)
- Methylphenidate (Ritalin, Concerta)
- Pseudoephedrine (Sudafed)
Other Weight Loss Medications
In most cases, it is not recommended to combine phentermine with other weight loss medications due to an increased risk of side effects [1.8.5]. Specifically, you should not take phentermine with Qsymia, as Qsymia already contains phentermine [1.3.2]. Caution is also advised with drugs like Contrave (bupropion/naltrexone), as bupropion can increase seizure risk when combined with phentermine [1.3.4].
| Interacting Drug Class | Examples | Potential Risk with Phentermine | Severity |
|---|---|---|---|
| MAOIs | Phenelzine (Nardil), Selegiline (Emsam) | Hypertensive Crisis (severe high blood pressure) | High (Contraindicated) |
| SSRIs/SNRIs | Sertraline (Zoloft), Duloxetine (Cymbalta) | Serotonin Syndrome | Moderate to High |
| Other Stimulants | Adderall, Ritalin, Sudafed | Increased cardiovascular side effects (heart rate, blood pressure) | High |
| Beta-blockers | Metoprolol (Lopressor), Atenolol (Tenormin) | May make beta-blockers less effective | Moderate |
| Diabetes Medications | Insulin, Glyburide (Diabeta) | Can increase the risk of low blood sugar (hypoglycemia) | Moderate |
| Alcohol | Beer, wine, spirits | Worsened side effects (dizziness, impaired judgment) | High |
Alcohol, Caffeine, and Supplements
Phentermine and Alcohol
It is strongly advised to avoid or limit alcohol use while taking phentermine [1.6.2]. Alcohol can intensify the side effects of phentermine, such as dizziness, drowsiness, and impaired concentration [1.6.2, 1.6.3]. Furthermore, phentermine is a stimulant that can mask the intoxicating effects of alcohol, making it difficult to gauge your level of impairment, which increases the risk of accidents and drinking more than intended [1.6.1].
Phentermine and Caffeine
Caffeine is also a stimulant. Consuming it with phentermine can lead to additive effects, increasing the risk of side effects like nervousness, palpitations, insomnia, and elevated heart rate and blood pressure [1.7.1, 1.7.4]. It is recommended to limit the intake of caffeine from sources like coffee, tea, chocolate, and energy drinks while on this medication [1.7.1, 1.7.3].
Herbal Supplements
Herbal supplements should also be used with caution. For example, St. John's wort can increase the risk of serotonin syndrome when taken with phentermine [1.2.1, 1.3.2]. Always inform your doctor about any herbs, vitamins, or supplements you are taking.
Conclusion
For safe and effective use, it is essential to be aware of what not to mix phentermine with. The most dangerous interaction is with MAOIs, which is an absolute contraindication. Significant risks also exist when combining phentermine with other antidepressants (SSRIs/SNRIs), stimulants, alcohol, and high amounts of caffeine. Always provide your healthcare provider with a complete list of all medications, supplements, and substances you use to avoid potentially harmful interactions. Your doctor can help determine if phentermine is a safe option for you based on your health history and current medications [1.8.3, 1.8.4].
Authoritative Link: For more detailed drug information, consult the U.S. National Library of Medicine's MedlinePlus resource on Phentermine [1.9.5].
