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Qsymia (Phentermine Hydrochloride / Topiramate) - Drug Interactions, Contraindications, Overdosage, etc



Monoamine Oxidase Inhibitors

Use of phentermine is contraindicated during or within 14 days following the administration of monoamine oxidase inhibitors because of the risk of hypertensive crisis.

Oral Contraceptives

Co-administration of multiple-dose Qsymia 15 mg/92 mg once daily with a single dose of oral contraceptive containing 35 g ethinyl estradiol (estrogen component) and 1 mg norethindrone (progestin component), in obese otherwise healthy volunteers, decreased the exposure of ethinyl estradiol by 16% and increased the exposure of norethindrone by 22% [see Clinical Pharmacology].

Although this study did not specifically address the impact of the interaction on contraceptive efficacy, an increased risk of pregnancy is not anticipated. The primary determinant of contraceptive efficacy is the progestin component of the combination oral contraceptive, so higher exposure to the progestin would not be expected to be deleterious.

However, irregular bleeding (spotting) may occur more frequently due to both the increased exposure to the progestin and lower exposure to the estrogen, which tends to stabilize the endometrium. Patients should be informed not to discontinue their combination oral contraceptive if spotting occurs, but to notify their health care provider if the spotting is troubling to them.

CNS Depressants Including Alcohol

Specific drug interaction studies of Qsymia and alcohol or other CNS depressant drugs have not been performed. The concomitant use of alcohol or CNS depressant drugs (e.g., barbiturates, benzodiazepines, and sleep medications) with phentermine or topiramate may potentiate CNS depression such as dizziness or cognitive adverse reactions, or other centrally mediated effects of these agents. Therefore, if Qsymia is used with alcohol or other CNS depressants, the patient should be counseled regarding possible increased risk of CNS depression or side effects.

Non-Potassium Sparing Diuretics

Concurrent use of Qsymia with non-potassium sparing diuretics may potentiate the potassium-wasting action of these diuretics. Concomitant administration of hydrochlorothiazide alone with topiramate alone has been shown to increase the Cmax and AUC of topiramate by 27% and 29%, respectively. When prescribing Qsymia in the presence of non-potassium-sparing medicinal products, patients should be monitored for hypokalemia [see Warnings and Precautions and Clinical Pharmacology].

Antiepileptic Drugs

Concomitant administration of phenytoin or carbamazepine with topiramate in patients with epilepsy, decreased plasma concentrations of topiramate by 48% and 40%, respectively, when compared to topiramate given alone [see Clinical Pharmacology].

Concomitant administration of valproic acid and topiramate has been associated with hyperammonemia with and without encephalopathy. Concomitant administration of topiramate with valproic acid in patients has also been associated with hypothermia (with and without hyperammonemia). It may be prudent to examine blood ammonia in patients in whom the onset of hypothermia or encephalopathy has been reported [see Clinical Pharmacology].

Carbonic Anhydrase Inhibitors

Concomitant use of topiramate, a component of Qsymia, with any other carbonic anhydrase inhibitor (e.g., zonisamide, acetazolamide, or dichlorphenamide) may increase the severity of metabolic acidosis and may also increase the risk of kidney stone formation. Avoid the use of Qsymia with other drugs that inhibit carbonic anhydrase [see Warnings and Precautions].


In the event of a significant overdose with Qsymia, if the ingestion is recent, the stomach should be emptied immediately by gastric lavage or by induction of emesis. Appropriate supportive treatment should be provided according to the patient's clinical signs and symptoms.

Acute overdose of phentermine may be associated with restlessness, tremor, hyperreflexia, rapid respiration, confusion, aggressiveness, hallucinations, and panic states. Fatigue and depression usually follow the central stimulation. Cardiovascular effects include arrhythmia, hypertension or hypotension, and circulatory collapse. Gastrointestinal symptoms include nausea, vomiting, diarrhea, and abdominal cramps. Fatal poisoning usually terminates in convulsions and coma. Manifestations of chronic intoxication with anorectic drugs include severe dermatoses, marked insomnia, irritability, hyperactivity and personality changes. A severe manifestation of chronic intoxication is psychosis, often clinically indistinguishable from schizophrenia.

Management of acute phentermine intoxication is largely symptomatic and includes lavage and sedation with a barbiturate. Acidification of the urine increases phentermine excretion. Intravenous phentolamine has been suggested for possible acute, severe hypertension, if this complicates phentermine overdosage.

Topiramate overdose has resulted in severe metabolic acidosis. Other signs and symptoms include convulsions, drowsiness, speech disturbance, blurred vision, diplopia, mentation impaired, lethargy, abnormal coordination, stupor, hypotension, abdominal pain, agitation, dizziness, and depression. The clinical consequences were not severe in most cases, but deaths have been reported after poly-drug overdoses involving gram amounts of topiramate. A patient who ingested a dose between 96 and 110 grams topiramate was admitted to hospital with coma lasting 20 to 24 hours followed by full recovery after 3 to 4 days.

Activated charcoal has been shown to adsorb topiramate in vitro. Hemodialysis is an effective means of removing topiramate from the body.


Qsymia is contraindicated in the following conditions:

  • Pregnancy [see Warnings and Precautions and Use in Specific Populations]
  • Glaucoma [see Warnings and Precautions]
  • Hyperthyroidism
  • During or within 14 days following the administration of monoamine oxidase inhibitors [see Drug Interactions]
  • Known hypersensitivity or idiosyncrasy to the sympathomimetic amines [see Adverse Reactions].


Controlled Substance

Qsymia is controlled in Schedule IV of the Controlled Substances Act because it contains phentermine a Schedule IV drug. Any material, compound, mixture, or preparation that contains any quantity of phentermine is controlled as a Schedule IV drug.

Topiramate is not controlled in the Controlled Substances Act.


Phentermine, a component of Qsymia, has a known potential for abuse.

Phentermine, a component of Qsymia, is related chemically and pharmacologically to the amphetamines. Amphetamines and other stimulant drugs have been extensively abused and the possibility of abuse of phentermine should be kept in mind when evaluating the desirability of including Qsymia as part of a weight reduction program. Abuse of amphetamines and related drugs (e.g., phentermine) may be associated with impaired control over drug use and severe social dysfunction. There are reports of patients who have increased the dosage of these drugs to many times that recommended.


Qsymia has not been systematically studied in for its potential to produce physical dependence. Physical dependence is a state that develops as a result of physiological adaptation in response to repeated drug use. Physical dependence manifests by drug-class-specific withdrawal symptoms after abrupt discontinuation or a significant dose reduction of a drug.

Limited information on the potential for physical dependence for the individual components of Qsymia is available. For topiramate, abrupt discontinuation has been associated with seizures in patients without a history of seizures or epilepsy. For phentermine, abrupt cessation following prolonged high dosage administration results in extreme fatigue and mental depression; changes are also noted on a sleep electroencephalogram. Thus, in situations where rapid withdrawal of Qsymia is required, appropriate medical monitoring is recommended.

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