Qsymia Drug Interactions: What to Avoid and What to Tell Your Doctor

Qsymia (phentermine/topiramate extended-release) can interact with a significant number of medications. These interactions can range from reducing the effectiveness of your other medications to increasing the risk of serious side effects. Before starting Qsymia, give your provider a complete list of everything you take—including prescription drugs, OTC medications, supplements, and herbal products.

Contraindicated Combinations: Never Take Together

The following combination is strictly contraindicated with Qsymia:

MAO inhibitors (MAOIs): Including phenelzine (Nardil), tranylcypromine (Parnate), selegiline (Eldepryl), isocarboxazid (Marplan), and linezolid. Do not take Qsymia if you have taken an MAOI within the past 14 days—this combination can cause hypertensive crisis or severe sympathomimetic reactions.

Major Interactions: Discuss With Your Doctor Before Combining

Valproic acid (Depakote): Combining topiramate with valproic acid can increase blood ammonia levels (hyperammonemia), which can progress to confusion and encephalopathy. If you take valproic acid for seizures or mood disorders, this combination requires careful monitoring.

Other carbonic anhydrase inhibitors: Zonisamide (Zonegran), acetazolamide (Diamox), or methazolamide should be avoided with Qsymia. Topiramate already inhibits carbonic anhydrase, and stacking these drugs greatly increases the risk of kidney stone formation and metabolic acidosis.

Lithium: Topiramate-induced metabolic acidosis can affect lithium levels and kidney function. If you take lithium for bipolar disorder, your provider should monitor your lithium levels and renal function closely.

Amitriptyline (and other tricyclic antidepressants): Topiramate can significantly increase amitriptyline levels in the blood. Your doctor may need to reduce your amitriptyline dose.

Moderate Interactions: Require Monitoring or Dose Adjustment

Oral contraceptives: Topiramate can reduce blood levels of estrogen and progestins in hormonal contraceptives, potentially reducing their effectiveness. This is especially important because Qsymia requires effective contraception due to fetal harm risk. Women on oral contraceptives must use a barrier method as additional protection, or discuss switching to a non-hormonal or intrauterine contraceptive method with their provider.

Antidiabetic medications: Qsymia-related weight loss can improve insulin sensitivity, potentially causing blood sugar to drop lower than expected. If you take insulin, metformin, sulfonylureas, or other diabetes medications, your provider may need to adjust your doses to prevent hypoglycemia.

CNS depressants (alcohol, benzodiazepines, opioids, sleep aids): Topiramate has CNS depressant effects. Combining Qsymia with alcohol or other CNS depressants can increase sedation, cognitive impairment, and psychomotor slowing. Avoid alcohol while on Qsymia. Use caution with benzodiazepines, sedatives, or opioids.

Diuretics (water pills): Topiramate's effects on kidney function can interact with diuretics like furosemide or hydrochlorothiazide, potentially affecting electrolyte balance. Potassium monitoring may be needed.

Antihypertensive medications: Phentermine can increase heart rate and blood pressure in some patients. If you're on blood pressure medications, your provider should monitor your response to Qsymia closely.

Ketogenic diet: Following a ketogenic (very low carbohydrate) diet while taking Qsymia significantly increases the risk of kidney stone formation. The combination of topiramate's carbonic anhydrase inhibition and the metabolic effects of ketosis creates a high-risk environment for stone development.

Drug-Related Heat Sensitivity

Topiramate can decrease sweating (oligohidrosis), increasing the risk of heat-related illness. This risk is greater when Qsymia is combined with other drugs that have anticholinergic effects (such as antihistamines, bladder medications, or certain antidepressants), or other carbonic anhydrase inhibitors. Be extra cautious exercising in heat.

What to Tell Your Doctor Before Starting Qsymia

Give your provider a complete medication list including:

All prescription medications, including psychiatric medications, seizure drugs, and blood pressure medications

Any OTC medications (especially decongestants, sleep aids, or diet pills)

All supplements and herbal products (some interact with either phentermine or topiramate)

Any diets you plan to follow (especially ketogenic or low-carb)

Your contraception method (critical for women who can become pregnant)

For a complete overview of Qsymia's side effects and warning signs, see our guide on Qsymia side effects. And if you're having trouble finding Qsymia at a pharmacy near you, medfinder can help locate a REMS-certified pharmacy that has your dose in stock.