Phentermine/Topiramate XR Drug Interactions

If you're taking Phentermine/Topiramate XR (Qsymia), it's important to know which other medications, supplements, and even foods can interact with it. Some interactions are dangerous, while others simply require a dose adjustment. Either way, your doctor and pharmacist need the full picture.

This guide covers the major and moderate drug interactions based on FDA labeling so you know what to avoid and what to discuss at your next appointment.

How Drug Interactions Work

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

• One drug makes another stronger (increasing side effects or toxicity)
• One drug makes another weaker (reducing its effectiveness)
• Two drugs together cause a new problem that neither would cause alone

Phentermine/Topiramate XR contains two active ingredients — Phentermine and Topiramate — which means it has interactions from both components. That's more interactions to keep track of than a single-ingredient medication.

Medications That Interact With Phentermine/Topiramate XR

Major Interactions (Avoid These)

• MAO inhibitors — Isocarboxazid (Marplan), Phenelzine (Nardil), Selegiline (Emsam), and Tranylcypromine (Parnate). Taking Phentermine/Topiramate XR within 14 days of an MAOI can cause a dangerous spike in blood pressure (hypertensive crisis). This combination is contraindicated.
• Other CNS depressants — Benzodiazepines (Alprazolam/Xanax, Diazepam/Valium), opioids (Oxycodone, Hydrocodone), sleep aids (Zolpidem/Ambien), and sedating antihistamines. The Topiramate component adds to sedation, increasing the risk of excessive drowsiness and cognitive impairment.
• Carbonic anhydrase inhibitors — Acetazolamide (Diamox) and Zonisamide (Zonegran). These medications combined with Topiramate significantly increase the risk of metabolic acidosis and kidney stones. Your doctor should monitor you closely if this combination is necessary.

Moderate Interactions (May Need Adjustment)

• Oral contraceptives — Topiramate can reduce the effectiveness of hormonal birth control pills, increasing the risk of breakthrough bleeding and unintended pregnancy. This is especially important since Phentermine/Topiramate XR causes birth defects. Talk to your doctor about using an additional or alternative form of contraception.
• Antidiabetic medications — Metformin, Insulin, Sulfonylureas (Glipizide, Glyburide). As you lose weight on Phentermine/Topiramate XR, your blood sugar may drop. Your doctor may need to reduce your diabetes medication dose to prevent hypoglycemia.
• Antihypertensives — Lisinopril, Amlodipine, Losartan, and other blood pressure medications. Weight loss can lower blood pressure naturally, so your blood pressure meds may need to be reduced.
• Valproic acid (Depakote) — Combining Valproic acid with Topiramate increases the risk of hypothermia (dangerously low body temperature) and metabolic acidosis. Valproic acid levels may also change.
• Lithium — Topiramate may alter Lithium blood levels, requiring more frequent monitoring.
• Other weight loss medications — Taking additional appetite suppressants or weight loss drugs alongside Phentermine/Topiramate XR is generally not recommended due to increased cardiovascular risk and side effects.

Supplements and OTC Medications to Watch

Don't assume that over-the-counter products are always safe to combine with Phentermine/Topiramate XR:

• Decongestants containing Pseudoephedrine or Phenylephrine (Sudafed, many cold medications) — These are also sympathomimetics and can add to the stimulant effects of Phentermine, increasing heart rate and blood pressure.
• Caffeine supplements and high-caffeine products — Caffeine combined with Phentermine can increase jitteriness, insomnia, and heart rate.
• St. John's Wort — May interact with the Topiramate component and affect its metabolism.
• Calcium and vitamin D supplements — Generally safe, but worth mentioning to your doctor since kidney stone risk is already elevated.

Food and Drink Interactions

• Alcohol — This is the big one. Alcohol combined with Phentermine/Topiramate XR increases central nervous system depression, which means worse dizziness, drowsiness, and cognitive impairment. It can also worsen mood-related side effects. Limit or avoid alcohol during treatment.
• Ketogenic (very low-carb) diets — A keto diet can worsen the metabolic acidosis risk associated with Topiramate. If you're following a very low-carb diet, let your doctor know so they can monitor your blood bicarbonate levels.

What to Tell Your Doctor

Before starting Phentermine/Topiramate XR, give your doctor a complete list of:

• All prescription medications you currently take
• Over-the-counter medications (including cold remedies and pain relievers)
• Vitamins and supplements
• Herbal products
• Your typical alcohol consumption
• Any dietary changes you're making (especially keto or very low-carb)

Also tell your doctor if you have a history of:

• Kidney stones
• Glaucoma
• Mood disorders (depression, anxiety, suicidal thoughts)
• Liver or kidney disease
• Heart problems

For a full overview of this medication, see What Is Phentermine/Topiramate XR? and for side effect details, read Phentermine/Topiramate XR side effects.

Final Thoughts

Because Phentermine/Topiramate XR contains two active ingredients, it has a broader interaction profile than many single-drug prescriptions. The most critical interactions to avoid are MAO inhibitors and other carbonic anhydrase inhibitors. But even everyday items like cold medicine, caffeine, and alcohol can matter.

The bottom line: keep your doctor and pharmacist fully informed about everything you're taking. And if you're looking for this medication, check availability on Medfinder.