Phentermine and Topiramate (Oral)

Phentermine and topiramate, also known as Qysmia, is an oral medicine which reduces appetite in order to aid weight loss.


Phentermine and topiramate is a combination oral medicine used to aid weight loss. It is usually prescribed to obese or overweight adults who have weight-related medical conditions, such as high blood pressure, high cholesterol and diabetes. However, it is only effective if used alongside a calorie-controlled diet and exercise regime. Not only does it help patients to lose weight, but it also helps to prevent them from regaining weight at a later point.

Phentermine is an anorectic and works to decrease appetite. Topiramate is an anticonvulsant and works to both decrease appetite and cause feelings of fullness to last longer. Together, these drugs can help patients to feel more satisfied after eating when following a calorie-restricted, portion-controlled diet.

In the US, phentermine and topiramate is administered in extended-release capsules which are known by the brand name Qysmia. The medicine is only available with a doctor's prescription and can only be obtained through certified pharmacies via a distribution program called Qysmia REMS. This is because the medicine poses a variety of serious side effects and the use of the drug must be closely monitored by a doctor.

Conditions Treated?

Type Of Medicine?

  • Anorectic (phentermine)
  • Anticonvulsant (topiramate)

Side Effects

Phentermine and topiramate can cause various unwanted effects as well as its side effects. Although it's unlikely you will experience all of these unwanted effects, you should still familiarize yourself with them in order that you can recognize serious side effects and seek medical attention where appropriate.

You should report any of the following serious side effects to your doctor immediately:

  • Less common
  • Muscle cramps or pain
  • Tingling or numbness in lips, hands or feet
  • Convulsions
  • Chest pain or discomfort
  • Fast, pounding or irregular heartbeat
  • Shortness of breath
  • Decreased urine output
  • Complete loss of appetite
  • Nausea or vomiting
  • Mood changes
  • Unusual tiredness or weakness
  • Rare
  • Pain in groin or genitals
  • Sharp pain in back below the ribs
  • Blood in urine
  • Incidence unknown
  • Blistering, peeling or loosening of the skin
  • Blisters on mouth, scalp, trunk or other areas
  • Red lesions on skin, often with purple center
  • Sores, ulcers or white spots on lips or in mouth
  • Itching
  • Swelling of face
  • Yellow eyes or skin
  • Red, irritated eyes
  • Blurred or reduced vision
  • Upper right or abdominal pain
  • Chest pain
  • Irregular heartbeat
  • Weak pulse
  • Cough or sore throat
  • Clumsiness
  • Joint or muscle pain
  • Muscle aches or weakness
  • Pain in shoulders, arms, neck or jaw
  • Shakiness in arms, hands, legs or feet
  • Increased frequency of seizures in patients with history of seizures or epilepsy
  • Tiredness and weakness
  • Chills or shivering
  • Low body temperature
  • Sweating
  • Headache
  • Difficulty thinking, speaking or walking
  • Dizziness
  • Persistent nausea or vomiting
  • Dark urine
  • Diarrhea
  • Light-colored stools
  • Weight gain

The following side effects are usually minor and do not require medical attention unless they become prolonged, persistent or severe. They may dissipate once your body adjusts to the medicine. If you have questions about them, consult your doctor.

  • More common
  • Back pain
  • Pain or tenderness around eyes or cheekbones
  • Change in taste
  • Loss of taste
  • Loss of voice
  • Difficulty passing stools
  • Ear congestion
  • Stuffy or runny nose
  • Sneezing
  • Nervousness or fear
  • Memory problems
  • Trouble sleeping
  • Less common
  • Belching
  • Heartburn or indigestion
  • Stomach upset, discomfort or pain
  • Decreased appetite
  • Cramps
  • Dry eyes
  • Eye pain
  • Hair loss
  • Heavy bleeding
  • Rash
  • Irritability
  • Incidence unknown
  • Bloating
  • Hives or welts
  • Change in sex drive, or sexual ability
  • Unusual behavior
  • False or unusual sense of well-being
  • Feeling that others are watching you or controlling you
  • Feeling that others can hear your thoughts
  • Feeling, seeing or hearing things that are not there (hallucinations)
  • Severe mood or mental changes

The side effects listed here may not be exhaustive of all unwanted effects that might be caused by phentermine and topiramate. If you notice any other side effects, report them to your doctor as soon as possible. You could also report them directly to the FDA, or your doctor may do this on your behalf.


The following are average doses for adults and should be used as a rough guideline only. You should always follow your doctor's dosing instructions, and never take more phentermine and topiramate than is prescribed to you, or take the capsules more often than directed.

  • Starting dose: 3.75 mg phentermine and 23 mg topiramate, taken once each day for 14 days
  • After 14 days: 7.5 mg phentermine and 46 mg topiramate, taken once each day
  • Maximum dose: 15 mg phentermine and 92 mg topiramate

Your doctor will monitor you regularly, particularly when you first start taking phentermine and topiramate, to assess your progress and check for unwanted side effects. They may periodically increase your dose to achieve the best results, but you may not necessarily need to take the maximum dose.

How to take phentermine and topiramate

Phentermine and topiramate capsules should be swallowed whole, without breaking, crushing or chewing. They should be taken in the morning and can be taken either with or without food. The medicine can have stimulating effects and can cause insomnia if taken close to bedtime, which is why morning doses are recommended.

You should not take phentermine and topiramate for longer than recommended by your doctor. This is because the drug can become habit-forming if taken in high quantities for long periods of time.

Your doctor will recommend a calorie-controlled diet and an exercise regime to ensure you successfully lose weight while taking phentermine and topiramate. It is very important that you follow your doctor's instructions to get the best possible results from this treatment. It's also vital to drink plenty of water while taking phentermine and topiramate because it will help to prevent kidney stones.

If you do not lose a certain amount of weight within the first 12 weeks of taking phentermine and topiramate, your doctor will probably tell you to stop taking the drug.

Missed dose

If you miss a dose of phentermine and topiramate, take it as soon as you remember unless it is almost time for your next dose. If this is the case, simply skipped the missed dose and continue with your original dosing schedule. Do not double doses of phentermine and topiramate as doing so could increase the risk of side effects.


There are many drugs which can interact with phentermine and topiramate and cause serious complications or side effects. It is incredibly important to tell your doctor about all the medicines you take in order that they can avoid causing harmful interactions. You should mention both the prescription medicines and over-the-counter medicines you take, as well as any herbal supplements or multivitamins, or other weight loss aids.

It is particularly important that your doctor knows you are taking any of the following types of medicines:

  • Anti-anxiety medicines
  • Anti-seizure medicines
  • Carbonic anhydrase inhibitors, such as:
  • Acetazolamide (Diamox)
  • Methazolamide
  • Zonisamide (Zonegran)
  • Diuretics (water pills), such as:
  • Furosemide (Lasix)
  • Hydrochlorothiazide (HCTZ)
  • High blood pressure medicines
  • Insulin or other diabetes medicines
  • Irritable bowel disease (IBD) medicines
  • Medicines for mental illness
  • Medicines for ulcers
  • Medicines for urinary problems
  • Monoamine oxidase inhibitors (MAOIs), such as:
  • Isocarboxazid (Marplan)
  • Phenelzine (Nardil)
  • Selegiline (Eldepryl, Emsam, Zelapar)
  • Tranylcypromine (Parnate)
  • Motion sickness medicines
  • Parkinson's disease medicines
  • Sedatives
  • Sleeping aids
  • Tranquilizers

The following medicines should never be taken with phentermine and topiramate due to the risk of dangerous interaction. If you currently take any of them, your doctor will either not prescribe phentermine and topiramate, or they will change some of the medicines you are already taking.

  • Furazolidone
  • Iproniazid
  • Isocarboxazid (Marplan)
  • Linezolid (Zyvox)
  • Methylene Blue (Provayblue, Urolene Blue)
  • Moclobemide (Manerix)
  • Nialamide
  • Phenelzine (Nardil)
  • Procarbazine (Matulane)
  • Rasagiline (Azilect)
  • Safinamide (Xadago)
  • Selegiline (Eldepryl, Emsam, Zelapar)
  • Sibutramine (Meridia)
  • Tranylcypromine (Parnate)

Where less serious interactions occur, doctors may either change some of the medicines you already take or, if both drugs are deemed important, prescribe both with dosage adjustments. Your doctor may ask you to take certain drugs at certain times of the day in order to minimize interactions. It is incredibly important that you follow your doctor's instructions closely, as failure to do so could cause serious side effects or affect the way phentermine and topiramate is absorbed by the body, which may make it less effective.


Avoid ketogenic diets

Ketogenic diets those which are high in protein and fat and low in carbohydrates can increase the risk of kidney stones when followed while taking phentermine and topiramate. You should ask your doctor for advice on creating a balanced, safe and healthy diet that will aid weight loss throughout treatment with phentermine and topiramate.

Mental health

Phentermine and topiramate can cause changes to mental health, mood and behavior. In clinical trials, a small number of patients became suicidal. Make sure your doctor knows your full medical history; if you currently suffer from depression or suicidal ideation, or have done in the past, you may not be able to take phentermine and topiramate or your doctor may want to monitor you more closely when you first start taking it.

If you notice that your mental health deteriorates while taking phentermine and topiramate, contact your doctor. You should report any of the following symptoms to your doctor immediately:

  • Anxiety
  • Panic attacks
  • Agitation or restlessness
  • New or worsening irritability, aggression or hostility
  • Impulsiveness
  • Depression
  • Mania (frenzied, abnormally excited mood)
  • Thoughts or actions of self-harm
  • Suicidal thoughts or actions
  • Withdrawing from family and friends

Contraindicated conditions

It's important that your doctor knows about your full medical history, because some conditions may be worsened by phentermine and topiramate, or they may increase the risk or severity of certain side effects.

People with the following conditions should not take phentermine and topiramate:

Phentermine and topiramate is also not recommended for people with unstable heart or blood vessel disease, but doctors may continue to prescribe it if the benefits of the drug far outweigh potential risks.

The following conditions may be worsened by phentermine and topiramate:

  • Bone problems, such as osteoporosis
  • Eye or vision problems
  • Depression (including a history of)
  • Mental illness (including a history of)
  • Drug abuse or dependency (including a history of)
  • Hypokalemia (low blood potassium levels)
  • Metabolic acidosis (too much acid in the blood) (including a history of)
  • Kidney stones
  • Seizures or epilepsy (including a history of)

People with the following conditions may develop metabolic acidosis (too much acid in the blood) when taking phentermine and topiramate:

  • Diarrhea
  • Lung disease or other breathing problems
  • Kidney problems
  • Status epilepticus (state of epilepsy in which multiple seizures occur without consciousness in between)
  • Surgery

People with moderate or severe kidney disease or moderate liver disease may not be able to take phentermine and topiramate. This is because the drug will be removed from the body at a much slower rate than normal, which means each dose will be more potent and could cause more severe side effects. If doctors choose to prescribe the medicine, they may administer lower doses to compensate for this, and they may require closer monitoring of the patient.

Risk of drowsiness

Phentermine and topiramate may cause blurred vision and can cause drowsiness, impaired thinking and impaired reactions. You should therefore avoid driving or operating machinery until you know how the drug affects you.

Do not take other diet aids

Other diet aids, including herbal supplements, may interact with phentermine and topiramate and cause serious side effects or health complications. Do not take any other weight-loss products without first getting approval from your doctor.

Pregnancy and breastfeeding

Phentermine and topiramate is contraindicated for use during pregnancy. Studies have demonstrated evidence of fetal abnormalities, particularly cleft lip or palate. This birth defect can occur even in the very earliest stages of pregnancy and perhaps even before the pregnancy is detected. Women should use appropriate contraception to prevent pregnancy while taking this medicine. If they do become pregnant, they should immediately stop taking the medicine and consult their doctor.

Although there have not been studies into whether phentermine and topiramate combination is excreted into human breast milk, it is known that each drug is excreted into breast milk when taken individually. Both drugs may be harmful to nursing infants. Patients should therefore either choose to stop breastfeeding or choose not to take phentermine and topiramate.


If you have had an allergic reaction to phentermine and topiramate in the past, or to similar drugs (such as adrenaline, amphetamine, dopamine, dobutamine, ephedrine or lisdexamfetamine), you may not be able to take this medicine. Make sure your doctor knows about all allergies you suffer from, including food, chemical, pollen and drug allergies, so that they can check you're not allergic to any of the ingredients in phentermine and topiramate.

Call your doctor or seek immediate medical attention if you experience any of the following signs of allergic reaction when taking phentermine and topiramate:

  • Hives or rash
  • Swelling of face, lips, tongue or throat
  • Difficulty breathing

Avoid alcohol

Drinking alcohol may increase the risk or severity of side effects of phentermine and topiramate. You should avoid alcohol while taking this medicine. Consult your doctor if you feel you may struggle to avoid alcohol completely.


Store phentermine and topiramate in the container it was provided with the lid tightly closed at all times. Keep it out of sight and reach of children and away from pets. It should be stored at room temperature and away from direct light, heat, and moisture. Do not store it in the bathroom and keep it from freezing.

If you have unused or expired phentermine and topiramate, dispose of it safely. Your healthcare provider will be able to tell you the most appropriate way to do this. Do not flush the capsules down the toilet or throw them in the trash. There may be a medicine take-back program run by your local recycling department, your pharmacy or your healthcare provider which will allow you to dispose of the medicine without it causing harm to the environment or to other people.


Phentermine and topiramate is a weight loss aid which works to decrease appetite and increase feelings of fullness after eating. It is usually only prescribed to obese or overweight patients who have weight-related medical conditions, and need help to adhere to a calorie-controlled diet in order to successfully lose weight and avoid weight regain. It is strictly controlled via the Qysmia REMS program and can only be purchased from approved pharmacies with a doctor's prescription.

The drug can cause mood and behavior changes, particularly in patients with a history of mental health problems. It is not suitable for people with glaucoma or hyperthyroidism, or those with unstable heart or blood vessel disease. It is also contraindicated for use during pregnancy and women should use birth control to prevent pregnancy while taking it.