phentermine/topiramate (Rx)

Brand and Other Names:Qsymia
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Dosing & Uses


Dosing Forms & Strengths

Phentermine (immediate-release)/topiramate (extended-release)

capsule: Schedule IV

  • 3.75mg/23mg
  • 7.5mg/46mg
  • 11.25mg/69mg
  • 15mg/92mg

Weight Managetment

Adjunct to reduced-calorie diet and exercise

Initial BMI ≥30 kg/m² (obese), or BMI ≥27 kg/m² (overweight) in presence of at least 1 weight related comorbidity (eg, hypertension, type 2 diabetes mellitus, dyslipidemia)

Initial: 3.75 mg/23 mg PO qDay for 14 days, THEN

Day 15: Increase to 7.5 mg/46 mg PO qDay for 12 weeks, then evaluate weight loss

If a patient has not lost at least 3% of baseline body weight on 7.5 mg/46 mg, discontinue or escalate dose, as it is unlikely that the patient will achieve and sustain clinically meaningful weight loss at the 7.5 mg/46 mg dose

To escalate the dose: Increase to 11.25 mg/69 mg PO qDay for 14 days; followed by dosing 15 mg/92 mg qDay; evaluate weight loss following dose escalation to 15 mg/92 mg after an additional 12 weeks of treatment

If a patient has not lost at least 5% of baseline body weight on 15 mg/92 mg, discontinue as directed (ie, gradually), as it is unlikely that the patient will achieve and sustain clinically meaningful weight loss with continued treatment

Note: 3.75 mg/23 mg and 11.25 mg/69 mg dosage strengths are for titration purposes only


Take once daily in the morning with or without food

Avoid dosing in the evening due to the possibility of insomnia

Discontinue 15 mg/92 mg gradually by taking a dose every other day for at least 1 week prior to stopping treatment altogether, due to the possibility of precipitating a seizure

Renal & Hepatic Impairment

Renal impairment

  • Mild (≥50 mL/min): No dose adjustment required
  • Moderate (30-49 mL/min) and severe (<30 mL/min): Not to exceed 7.5 mg/46 mg qDay

Hepatic impairment

  • Mild (Child-Pugh 5-6): No dose adjustment required
  • Moderate (Child-Pugh 7-9): Not to exceed 7.5 mg/46 mg qDay
  • Severe (Child-Pugh 10-15): Avoid use

Safety and efficacy not established



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            Adverse Effects


            Paresthesia (4.2-19.9%)

            Dry mouth (6.7-19.1%)

            Constipation (7.9-16.1%)

            URI (12.2-15.8%)

            Metabolic acidosis (6.4-12.8%)

            Nasopharyngitis (9.4-12.5%)

            Headache (7-10.6%)


            Insomnia (5-9.4%)

            Dysgeusia (1.3-9.4%)

            Dizziness (2.9-8.6%)

            Increased serum creatinine (2.1-8.4%)

            Sinusitis (6.8-7.8%)

            Nausea (3.6-7.2%)

            Bronchitis (4.4-6.7%)

            Back pain (5.4-6.6%)

            Diarrhea (5-6.4%)

            Blurred vision (4-6.3%)

            Fatigue (4.4-5.9%)

            UTI (3.3-5.2%)

            Hypokalemia (persistent) (0.4-4.9%)

            Cough (3.3-4.8%)

            Depression (2.8-4.3%)

            Anxiety (1.8-4.1%)

            Irritability (1.7-3.7%)

            Alopecia (1.7-3.7%)

            Hypoesthesia (0.8-3.7%)

            Disturbance in attention (0.4-3.5%)

            GERD (0.8-3.2%)

            Extremity pain (2.1-3%)

            Musculoskeletal pain (0.8-3%)

            Muscle spasms (2.8-2.9%)

            Dyspepsia (2.1-2.8%)

            Sinus congestion (2-2.6%)

            Rash (1.7-2.6%)

            Pharyngeal pain (1.2-2.5%)

            Dry eye (0.8-2.5%)

            Gastroenteritis (0.8-2.5%)

            Hypokalemia (acute) (0.4-2.5%)

            Palpitations (0.8-2.4%)

            Eye pain (2.1-2.2%)

            Neck pain (1.2-2.2%)

            Oral paresthesia (0.2-2.2%)

            Decreased appetite (1.5-2.1%)

            Nephrolithiasis (0.2-1.2%)

            Dysmenorrhea (0.4-2.1%)

            Chest discomfort (0.2-2.1%)

            Thirst (1.8-2%)

            Nasal congestion (1.2-2%)

            Postmarketing Reports


            • Allergic: Urticaria
            • Cardiovascular: Increased blood pressure, ischemic events
            • CNS: Euphoria, psychosis, tremor
            • Reproductive: Changes in libido, impotence


            • Central nervous system: Suicidal behavior and ideation
            • Dermatologic: Bullous skin reactions (eg, erythema multiforme, Stevens-Johnson syndrome, toxic epidermal necrolysis), pemphigus
            • Gastrointestinal: Pancreatitis
            • Hepatic: Hepatic failure (including fatalities), hepatitis
            • Metabolic: Hyperammonemia, hypothermia
            • Ophthalmic: Maculopathy, angle closure glaucoma, increased intraocular pressure



            Hypersensitivity or idiosyncrasy to sympathomimetic amines




            Within 14 days following MAOIs (risk hypertensive crisis)


            May cause fetal harm (see Contraindications; see Pregnancy & Lactation)

            May increase resting heart rate up to 20 bpm; reduce dose or discontinue use with sustained increase in resting heart rate; caution in patients with history of cardiac or cerebrovascular disease

            Antiepileptic drugs (AEDs), including topiramate, increase the risk of suicidal thoughts or behavior

            Acute myopia associated with secondary angle closure glaucoma has been reported with topiramate; monitor for increased IOP due to risk of permanent loss of vision; discontinue therapy in patients with ocular pain or acute onset of decreased visual acuity

            May cause mood disorders, including depression, and anxiety, as well as insomnia; patient with history of depression may be at increased risk of recurrent depression or other mood disorders

            Cognitive dysfunction (eg, impairment of concentration/attention, difficulty with memory, and speech or language problems, particularly word-finding difficulties) reported; rapid titration or high initial doses may be associated with higher rates; may need to reduce dose or discontinue therapy

            Hyperchloremic, nonanion gap, and metabolic acidosis reported; monitor serum electrolytes and bicarbonate prior to and during treatment; reduce dose or discontinue use if persistent metabolic acidosis develops

            Increase in serum creatinine that reflects a decrease in renal function (glomerular filtration rate) reported; peak increases were observed in clinical trials after 4-8 weeks of treatment and gradually declined but remained elevated over baseline values

            Weight loss may increase the risk of hypoglycemia in patients with type 2 diabetes mellitus treated with insulin and/or insulin secretagogues; monitor for needed diabetic therapy adjustments

            In patients treated for hypertension, weight loss may increase the risk of hypotension; monitor for needed antihypertensive dose adjustments

            Coadministration with alcohol or CNS depressants drugs with phentermine or topiramate may potentiate CNS depression or other centrally mediated effects of these agents (eg, dizziness, cognitive adverse reactions, drowsiness, light-headedness, impaired coordination, somnolence)

            Abrupt withdrawal of topiramate associated with seizures in individuals without a history of seizures or epilepsy; gradually discontinue if taking 15 mg/92 mg

            Caution with renal or hepatic impairment and adjust dose (see Renal & Hepatic Impairment)

            Kidney stone formation reported; topiramate inhibits carbonic anhydrase activity and promotes kidney stone formation by reducing urinary citrate excretion and increasing urine pH; avoid coadministration with other carbonic anhydrase inhibitors; use in patients on a ketogenic diet may also increase this risk; increase fluid intake to decrease risk

            Oligohidrosis (resulting in hyperthermia) reported with topiramate; use caution and monitor patients receiving carbonic anhydrase inhibitors and drugs with anticholinergic effects, or during exposure to high environmental temperatures or stenuous exercise

            Can cause changes in laboratory values; can increase the risk of hypokalemia through its inhibition of carbonic anhydrase activity; caution with coadministration of drug that cause hypokalemia (eg, furosemide, hydrochlorothiazide)

            Obtain blood chemistry profile that includes bicarbonate, creatinine, potassium, and glucose at baseline and periodically during treatment

            Use with combination oral contraceptives may increase frequency of irregular bleeding/spotting; need not discontinue unless spotting is trouble to patient

            Consider dose reduction in patients with hepatic impairment; use with caution


            Pregnancy & Lactation


            Contraindicated in pregnant patients; use can cause fetal harm and weight loss offers no clear clinical benefit to a pregnant patient; available data from pregnancy registry and epidemiologic studies indicate increased risk in oral clefts (cleft lip with or without cleft palate) with first trimester exposure to topiramate, a component of the drug combination

            Maternal obesity increases risk for congenital malformations, including neural tube defects, cardiac malformations, oral clefts, and limb reduction defects; in addition, weight loss during pregnancy may result in fetal harm; appropriate weight gain based on pre-pregnancy weight is currently recommended for all pregnant patients, including those who are already overweight or obese, due to obligatory weight gain that occurs in maternal tissues during pregnancy

            Therapy can cause metabolic acidosis; effect of topiramate-induced metabolic acidosis has not been studied in pregnancy; however, metabolic acidosis in pregnancy (due to other causes) can cause decreased fetal growth, decreased fetal oxygenation, and fetal death, and may affect the fetus’ ability to tolerate labor

            Therapy can cause fetal harm when administered to a pregnant patient; advise patients who can become pregnant to use effective contraception during therapy

            For patients taking combined oral contraceptives (COCs), therapy may cause irregular bleeding; advise patients not to discontinue taking their COC and to contact their healthcare provider

            Animal data

            • When phentermine and topiramate were co-administered to rats at doses of 3.75 and 25 mg/kg, respectively [approximately 2 times maximum recommended human dose (MRHD) based on area under the curve (AUC)], or at the same dose to rabbits (approximately 0.1 times and 1 time, respectively, the clinical exposures at the MRHD based on AUC) there were no drug-related malformations;
            • However, structural malformations, including craniofacial defects and reduced fetal weights occurred in offspring of multiple species of pregnant animals administered topiramate at clinically relevant doses; advise pregnant women of potential risk to a fetus


            Drug components are present in human milk; there are no data on effects of topiramate and phentermine on milk production; diarrhea and somnolence reported in breastfed infants with maternal use of topiramate; there are no data on effects of phentermine in breastfed infants

            Because of potential for serious adverse reactions, including changes in sleep, irritability, hypertension, vomiting, tremor and weight loss in breastfed infants with maternal use of phentermine, advise patients that breastfeeding is not recommended during therapy

            Pregnancy Categories

            A: Generally acceptable. Controlled studies in pregnant women show no evidence of fetal risk.

            B: May be acceptable. Either animal studies show no risk but human studies not available or animal studies showed minor risks and human studies done and showed no risk.

            C: Use with caution if benefits outweigh risks. Animal studies show risk and human studies not available or neither animal nor human studies done.

            D: Use in LIFE-THREATENING emergencies when no safer drug available. Positive evidence of human fetal risk.

            X: Do not use in pregnancy. Risks involved outweigh potential benefits. Safer alternatives exist.

            NA: Information not available.



            Mechanism of Action

            Phentermine: Sympathomimetic amine; induces anorectic effect via release of norepinephrine in the hypothalamus causing appetite suppression by increasing blood leptin concentration; exact mechanism of action unknown

            Topiramate: Effect may be through appetite suppression and satiety enhancement that is induced by a combination of pharmacologic effects including augmenting neurotransmitter gamma-aminobutyrate activity, voltage-gated ion channels modulation, AMPA/kainite excitatory glutamate receptor inhibition, or carbonic anhydrase inhibition


            Phentermine (15 mg dose)

            • Peak Plasma Time: 6 hr
            • Peak Plasma Concentration: 49.1 ng/mL
            • AUC: 1990-2000 ng???hr/mL

            Topiramate (92 mg dose)

            • Peak Plasma Time: 9 hr
            • Peak Plasma Concentration: 1020 ng/mL
            • AUC: 61,600-68,000 ng???hr/mL



            • Protein Bound: 17.5%
            • Vd: 348 L


            • Protein Bound: 15-41%
            • Vd: 50.8 L (central compartment); 13.1 L (peripheral compartment)



            • Metabolized by 2 metabolic pathways, p-hydroxylation on the aromatic ring and N-oxidation on the aliphatic side chain
            • CYP3A4 primarily metabolizes phentermine but does not show extensive metabolism
            • Monoamine oxidase (MAO)-A and MAO-B do not metabolize phentermine


            • Not extensively metabolized



            • Half-life: 20 hr (terminal)
            • Clearance: 8.79 L/hr
            • Excretion: 70-80% urine (as unchanged drug)


            • Half-life: 65 hr (terminal)
            • Clearance: 1.17 L/hr
            • Excretion: 70% urine (as unchanged drug)




            FormularyPatient Discounts

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            The above information is provided for general informational and educational purposes only. Individual plans may vary and formulary information changes. Contact the applicable plan provider for the most current information.

            Tier Description
            1 This drug is available at the lowest co-pay. Most commonly, these are generic drugs.
            2 This drug is available at a middle level co-pay. Most commonly, these are "preferred" (on formulary) brand drugs.
            3 This drug is available at a higher level co-pay. Most commonly, these are "non-preferred" brand drugs.
            4 This drug is available at a higher level co-pay. Most commonly, these are "non-preferred" brand drugs or specialty prescription products.
            5 This drug is available at a higher level co-pay. Most commonly, these are "non-preferred" brand drugs or specialty prescription products.
            6 This drug is available at a higher level co-pay. Most commonly, these are "non-preferred" brand drugs or specialty prescription products.
            NC NOT COVERED – Drugs that are not covered by the plan.
            Code Definition
            PA Prior Authorization
            Drugs that require prior authorization. This restriction requires that specific clinical criteria be met prior to the approval of the prescription.
            QL Quantity Limits
            Drugs that have quantity limits associated with each prescription. This restriction typically limits the quantity of the drug that will be covered.
            ST Step Therapy
            Drugs that have step therapy associated with each prescription. This restriction typically requires that certain criteria be met prior to approval for the prescription.
            OR Other Restrictions
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            Medscape prescription drug monographs are based on FDA-approved labeling information, unless otherwise noted, combined with additional data derived from primary medical literature.