Know What Interacts With Adipex-P Before You Start

Adipex-P (Phentermine) is generally safe when used as prescribed, but it can interact with other medications, supplements, and even certain foods. Some of these interactions are minor. Others can be dangerous — even life-threatening.

Before starting Adipex-P, it's critical to tell your doctor about everything you're taking. This guide covers the most important interactions you should know about.

How Drug Interactions Work

A drug interaction happens when one substance changes how another substance works in your body. This can happen in several ways:

• Increased effects — One drug amplifies the effects of another, increasing the risk of side effects
• Decreased effects — One drug reduces how well another drug works
• New side effects — The combination creates side effects that neither drug causes alone

Because Phentermine is a stimulant that affects your heart rate, blood pressure, and nervous system, most of its interactions involve other drugs that affect these same systems.

Major Drug Interactions (Avoid These Combinations)

The following interactions are considered serious or potentially dangerous. In most cases, your doctor will avoid prescribing Adipex-P if you're taking any of these:

MAO Inhibitors — Contraindicated

This is the most dangerous interaction. Do not take Adipex-P during or within 14 days of using an MAO inhibitor. The combination can cause a hypertensive crisis — a sudden, severe spike in blood pressure that can lead to stroke or death.

MAO inhibitors include:

• Phenelzine (Nardil)
• Tranylcypromine (Parnate)
• Isocarboxazid (Marplan)
• Selegiline (Emsam, Zelapar) at higher doses
• Linezolid (Zyvox) — an antibiotic that also has MAO-inhibiting properties

Other CNS Stimulants and Weight Loss Drugs

Taking Adipex-P with other stimulants or appetite suppressants increases the risk of dangerous cardiovascular effects, including rapid heart rate, high blood pressure, and cardiac events. Avoid combining with:

• Amphetamine-based medications (Adderall, Vyvanse, Dexedrine)
• Methylphenidate (Ritalin, Concerta)
• Other prescription weight loss drugs (unless specifically directed by your doctor)

Serotonergic Drugs (SSRIs, SNRIs, and Others)

Combining Phentermine with drugs that increase serotonin levels may increase the risk of serotonin syndrome — a potentially life-threatening condition. Symptoms include agitation, confusion, rapid heart rate, high blood pressure, dilated pupils, muscle twitching, and fever.

Common serotonergic medications include:

• SSRIs: Fluoxetine (Prozac), Sertraline (Zoloft), Escitalopram (Lexapro), Paroxetine (Paxil), Citalopram (Celexa)
• SNRIs: Venlafaxine (Effexor), Duloxetine (Cymbalta), Desvenlafaxine (Pristiq)
• Triptans: Sumatriptan (Imitrex) and other migraine medications

Note: Many patients do take SSRIs and Phentermine together under close medical supervision. Your doctor will weigh the risks and benefits. The key is that they need to know about it.

Moderate Drug Interactions

These interactions may require dose adjustments or closer monitoring:

Tricyclic Antidepressants (TCAs)

TCAs like Amitriptyline (Elavil), Nortriptyline (Pamelor), and Imipramine (Tofranil) may increase or decrease the effects of Phentermine. Your doctor may need to adjust dosing if you're taking both.

Blood Pressure Medications

Phentermine can raise blood pressure, which may work against your blood pressure medication. Drugs like Guanethidine and other adrenergic neuron blocking agents may be less effective when taken with Phentermine. Your doctor should monitor your blood pressure more frequently if you're on both.

Insulin and Oral Diabetes Medications

As you lose weight on Adipex-P, your blood sugar levels may improve — which means your diabetes medications may need to be adjusted downward to avoid hypoglycemia (low blood sugar). Common diabetes medications that may need adjustment include:

• Insulin (all types)
• Metformin (Glucophage)
• Glipizide (Glucotrol)
• Glyburide (Diabeta)

This is actually a positive interaction — weight loss improving blood sugar — but it requires monitoring.

Supplements and Over-the-Counter Medications to Watch

It's not just prescription drugs you need to worry about. Some common supplements and OTC products can also interact with Adipex-P:

• Decongestants containing Pseudoephedrine or Phenylephrine (Sudafed, many cold and flu products) — These are also stimulants and can compound Phentermine's effects on heart rate and blood pressure.
• Caffeine pills or high-caffeine supplements — Extra caffeine on top of Phentermine can cause jitteriness, rapid heart rate, anxiety, and insomnia.
• St. John's Wort — This herbal supplement affects serotonin levels and may increase the risk of serotonin syndrome when combined with Phentermine.
• Weight loss supplements — Many over-the-counter weight loss products contain stimulants (like synephrine or bitter orange extract) that can dangerously amplify Phentermine's effects.

Food and Drink Interactions

Alcohol

The Adipex-P label warns that alcohol may cause adverse reactions when combined with Phentermine. Alcohol can increase dizziness, impair judgment, and put additional stress on your cardiovascular system. It's best to limit or avoid alcohol while taking Adipex-P.

Caffeine

While not contraindicated, caffeine can amplify stimulant side effects like increased heart rate, jitteriness, anxiety, and difficulty sleeping. You don't necessarily need to quit coffee, but consider cutting back — especially if you're experiencing these side effects.

Acidic and Alkaline Foods

Highly acidic foods and drinks (like citrus juice or vitamin C supplements) may increase how quickly your body eliminates Phentermine, potentially reducing its effectiveness. Alkaline foods may slow elimination and increase drug levels. This interaction is generally minor but worth knowing about.

What to Tell Your Doctor

Before starting Adipex-P, give your doctor a complete list of everything you take, including:

• All prescription medications (including those prescribed by other doctors)
• Over-the-counter medications (pain relievers, cold medicines, allergy medications)
• Vitamins and supplements (including herbal products)
• Any recreational substances

Also tell your doctor about:

• Any history of heart disease, high blood pressure, or stroke
• Any history of drug or alcohol abuse
• Whether you're pregnant, planning to become pregnant, or breastfeeding
• Any history of glaucoma, hyperthyroidism, or anxiety disorders

If you start any new medication while taking Adipex-P, let your prescriber know immediately — even if it seems unrelated.

Final Thoughts

Adipex-P is a safe and effective weight loss medication when used properly, but like all drugs, it doesn't exist in a vacuum. Being upfront with your doctor about everything you take is the single most important thing you can do to avoid dangerous interactions.

For more information about Adipex-P, check out our guides on side effects and uses, dosage, and what you need to know. If you need to fill your prescription, Medfinder can help you find a pharmacy with Adipex-P in stock near you.