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The market for products, supplements and drugs (prescription and otherwise) for weight loss have been growing rapidly in recent years with thousands of different options now readily available throughout the weight loss industry. Many of these products are available to purchase online or in health food stores nationwide; some are only available through a prescription from a qualified practicing physician.

The safest products to use are generally those prescribed by your doctor as part of a monitored weight loss plan. These come with a minimal risk to your health due to being tested in many clinical trials, and they can have enormous health benefits for the user. When combined with regular exercise and a healthy, well-balanced diet, it can become much easier to obtain and maintain your ideal weight.

Phentermine/Topiramate is one example of these prescription medications and this article will provide you with all of the information you need to understand what it is, what it does, the potential side effects and if it is right for you. This information is not a substitute for doctors’ advice so please book an appointment with your physician to discuss your options before embarking on any weight loss plan or taking new supplements or drugs.

What Is Phentermine/Topiramate? 

Phentermine/Topiramate are two separate drugs that are used in combination as an aid for weight loss.

Phentermine belongs to a group of psychostimulants known as ‘substituted amphetamines’, and is an appetite suppressant which is classified as a Schedule IV controlled substance in the USA.

Topiramate belongs to a group of drugs known as anticonvulsants, which help to decrease abnormal brain activity and is also used to treat epilepsy and migraines as well as the symptoms of eating disorders. As well as the active ingredient, Topiramate contains carnauba wax; polyethylene glycol; polysorbate 80; pregelatinized starch; purified water; sodium starch glycolate; synthetic iron oxide and titanium dioxide.

Together, Phentermine/Topiramate are used in the branded drug Qsymia as an aid for weight loss, which can be prescribed alongside a calorie restricted diet to patients who have not responded to a change of diet and exercise routine alone.

What Does Phentermine/Topiramate Do?

Phentermine/Topiramate, when consumed together, is an effective medication that can be prescribed by a physician to promote weight loss. Individually Topiramate can also be used to treat severe headaches and migraines as well as epilepsy. [1]

How Does Phentermine/Topiramate Work?

Phentermine/Topiramate have separate roles when taken to induce weight loss. Phentermine is a known appetite suppressant that affects the central nervous system and works to stop people confusing thirst or boredom for physical hunger, hence its role in the treatment of binge eating disorders. Phentermine also has the effect of making people feel full for longer, reducing the likelihood of overeating. Topiramate is said to have anti-anxiety properties which can help prevent emotional eating and is thought to inhibit the parts of the brain that are responsible for food cravings. [2]

What Are The Side-Effects Of Phentermine/Topiramate?

Some people taking Phentermine/Topiramate for weight loss do so without experiencing any side effects; however the response varies individually. Talk to your doctor if you are concerned about the side effects you experience when taking Phentermine/Topiramate for weight loss.

Some potential side effects of taking Phentermine/Topiramate are: 

  • Dizziness and vertigo
  • A dry mouth
  • Difficulty falling asleep or staying asleep (insomnia)
  • Irritability and depression
  • Nausea, vomiting and diarrhea
  • Constipation
  • Addiction- Phentermine can cause a physical ‘high’ or dependency in large doses and so is unsuitable for long term use

Do Not Take Phentermine/Topiramate If:

  • You have suffered from renal (kidney) impairment or renal failure
  • If you are pregnant and/or breast-feeding: It is not recommended for use by pregnant or lactating women; talk to your doctor for safe weight loss alternatives if you are concerned about your health. Taking Phentermine while pregnant can lead to a cleft lip or cleft palate in babies.
  • If you’re a child under the age of 18: Phentermine/Topiramate for weight loss is not intended for use by persons under the age of 18, (although Topiramate may occasionally be prescribed for children aged 12 and over who suffer from migraines). Talk to your doctor about safe weight loss alternatives for children whose weight may cause health problems.

Phentermine/Topiramate Dosages

The suggested dosage of Phentermine/Topiramate depends on the BMI of the patient. A standard initial dose for a patient wishing to lose weight is one capsule containing 3.75 mg of Phentermine combined with 23 mg of Topiramate.

After 12 weeks if the patient has not lost at least 3% of their body weight, the recommended dosage increases to 11 mg of Phentermine and 69 mg of Topiramate.

If treatment at the second dosage is not successful, a final attempt of 15 mg of Phentermine and 92 mg of extended-release Topiramate may be attempted. If the patient does not lose a minimum of 5% of their body weight after 12 weeks at the maximum dose, treatment is to be discontinued by tapering off the medication (taking one dose, every other day for at least one week).

Tapering off the medication is essential to prevent possible seizures. 

Is Phentermine/Topiramate Prescription-Only?

Phentermine/Topiramate are controlled substances, and people wishing to take the drug for weight loss purposes must obtain a prescription from a qualified medical practitioner.

Can I Take Phentermine/Topiramate With Other Medication?

There are some medications or drugs that should not be taken along with Phentermine/Topiramate. These include:

  • MAO inhibitors such as linezolid, isocarboxazid, methylene blue, tranylcypromine, moclobemide, phenelzine, safinamide, selegiline, tranylcypromine, procarbazine or rasagiline. Most of these drugs should also be stopped for at least two weeks prior to taking Phentermine/Topiramate; consult your doctor for advice if you have recently used these medications.
  • Alcohol should not be consumed while you are taking Phentermine/Topiramate as it works to dull the central nervous system and can work to prevent the effects of the medication.
  • Any medications that cause drowsiness.

You should also advise your doctor if you are:

  • Taking oral birth control, as you may experience unusual spotting while taking the drug.
  • Planning to become pregnant in the near future
  • Breastfeeding
  • Taking medications for depression or anxiety, as Phentermine/Topiramate may affect your mental health

Can I Take Phentermine/Topiramate With Other Supplements?

If you are taking vitamins or health supplements please let your doctor know and seek advice before starting a course of Phentermine/Topiramate.

Is Phentermine/Topiramate Safe?

When prescribed as a monitored course of medication by a qualified medical practitioner, and when there are no contraindications present (such as pregnancy or taking MAOs inhibitors) Phentermine/Topiramate has been found to be a safe drug to take for moderately paced weight loss.

The drug has been through a number of clinical trials and is deemed as generally safe; however side effects are possible and consumers need to be aware and monitor themselves for potentially dangerous reactions. [3]

Has Phentermine/Topiramate Caused Deaths?

There have not been any deaths linked to this medication. However, an overdose could result in serious cardiovascular problems, other physical and mental problems or death.

Does Phentermine/Topiramate Actually Work?

In clinical studies, patients taking Phentermine/Topiramate for weight loss experienced a reduction in body weight of 8.6% to 9.4% more than those taking a placebo.

Two large clinical trials have been carried out and researchers found a dose-response relationship, meaning that those taking a larger dose of Phentermine/Topiramate experienced more weight loss than those taking a smaller dosage. [4]


Phentermine/Topiramate is a drug compound similar to amphetamine in that it stimulates the central nervous system, suppresses abnormal brain activity and can suppress appetite. In clinical trials the drug has delivered significant results in terms of some patients losing up to 9% of their overall body fat; however, this drug is only prescribed to patients with a body mass index of 30kg/m2 or over, and must be used in conjunction with a calorie restrictive (deficit) diet and regular exercise program.

This drug should only be prescribed by a physician if the patient has not responded to traditional dieting and exercise.

While it has been deemed safe to use over short periods of time to help combat obesity, taking Phentermine/Topiramate is not a long-term solution for weight loss. It is important to note that this drug also stimulates the central nervous systems which could lead to other adverse and as yet unknown side effects

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This article has been reviewed and fact-checked by a certified nutritionist, and only uses information from credible academic sources.