Reviewed on 4/25/2022

What Is Phentermine/Topiramate and How Does It Work?

Phentermine/topiramate is a combination prescription medication used for weight management in adjunct to exercise and a reduced-calorie diet.

  • Phentermine/topiramate is available under the following different brand names: Qsymia

What Are Dosages of Phentermine/Topiramate?

Adult dosage

Capsule: Schedule IV

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

Weight Management

Adult and geriatric dosage

  • Initial: 3.75 mg/23 mg orally every day for 14 days, THEN
  • Day 15: Increase to 7.5 mg/46 mg orally every day 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 the 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 orally every day for 14 days; followed by dosing 15 mg/92 mg every day; 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 (i.e., gradually), as it is unlikely that the patient will achieve and sustain clinically meaningful weight loss with continued treatment

Dosage Considerations – Should be Given as Follows: 

  • See “Dosages”

What Are Side Effects Associated with Using Phentermine/Topiramate?

Common side effects of Phentermine/Topiramate include:

  • dizziness;
  • constipation;
  • numbness or tingly feeling;
  • sleep problems (insomnia); or
  • dry mouth, changes in your sense of taste.

Serious side effects of Phentermine/Topiramate include:

  • unusual changes in mood or behavior;
  • confusion, trouble concentrating, problems with speech or memory;
  • fast or pounding heartbeats while resting;
  • a seizure;
  • signs of a kidney stone--severe pain in your side or lower back, painful or difficult urination; or
  • signs of too much acid in the blood--loss of appetite, tiredness, thinking problems, irregular heartbeats.

Rare side effects of Phentermine/Topiramate include:

  • none 
This is not a complete list of side effects and other serious side effects or health problems that may occur as a result of the use of this drug. Call your doctor for medical advice about serious side effects or adverse reactions. You may report side effects or health problems to FDA at 1-800-FDA-1088.


The Best Diet Tips: How to Lose Weight the Healthy Way See Slideshow

What Other Drugs Interact with Phentermine/Topiramate?

If your medical doctor is using this medicine to treat your pain, your doctor or pharmacist may already be aware of any possible drug interactions and may be monitoring you for them. Do not start, stop, or change the dosage of any medicine before checking with your doctor, health care provider, or pharmacist first.

This information does not contain all possible interactions or adverse effects. Visit the RxList Drug Interaction Checker for any drug interactions. Therefore, before using this product, tell your doctor or pharmacist about all the products you use. Keep a list of all your medications with you and share this information with your doctor and pharmacist. Check with your health care professional or doctor for additional medical advice, or if you have health questions or concerns.

What Are Warnings and Precautions for Phentermine/Topiramate?


  • Hypersensitivity or idiosyncrasy to sympathomimetic amines
  • Pregnancy
  • Glaucoma
  • Hyperthyroidism
  • Within 14 days following MAOIs (risk hypertensive crisis)

Effects of drug abuse

  • None

Short-Term Effects

  • See “What Are Side Effects Associated with Using Phentermine/Topiramate?”

Long-Term Effects

  • See “What Are Side Effects Associated with Using Phentermine/Topiramate?”


  • May cause fetal harm (see Contraindications; see Pregnancy & Lactation)
  • May increase resting heart rate up to 20 bpm; reduce the dose or discontinue use with a sustained increase in resting heart rate; caution in patients with a 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 the 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; patients with a 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) was reported; rapid titration or high initial doses may be associated with higher rates; may need to reduce the dose or discontinue therapy
  • Hyperchloremic, non-anion gap, and metabolic acidosis reported; monitor serum electrolytes and bicarbonate before and during treatment; reduce the dose or discontinue use if persistent metabolic acidosis develops
  • An increase in serum creatinine that reflects a decrease in renal function (glomerular filtration rate) was 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 (e.g., 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 the dose (see Renal & Hepatic Impairment)
  • Kidney stone formation was 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 the 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 strenuous exercise
  • Can cause changes in laboratory values; can increase the risk of hypokalemia through its inhibition of carbonic anhydrase activity; caution with the coadministration of drugs that cause hypokalemia (e.g., furosemide, hydrochlorothiazide)
  • Obtain a blood chemistry profile that includes bicarbonate, creatinine, potassium, and glucose at baseline and periodically during treatment
  • Use with combination oral contraceptives may increase the frequency of irregular bleeding/spotting; need not discontinue unless spotting is trouble for the patient
  • Consider dose reduction in patients with hepatic impairment; use with caution

Pregnancy and 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
  • Lactation
    • 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


Weight loss occurs in the belly before anywhere else. See Answer
Medscape. Phentermine-Topiramate.

Health Solutions From Our Sponsors