Phentermine vs Topiramate vs Qsymia for Weight Loss

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Phentermine and Qsymia are both medications used for weight loss. Phentermine is only approved for short-term usage, while Qsymia is approved for long-term use.

Topiramate is a medication used to treat seizures and prevent migraines. It also happens to have a noticeable weight loss effect, although it’s not approved in Canada solely for treating obesity.

But how do they work? And how do their weight loss effects compare to each other?

Let’s compare the three drugs phentermine, topiramate, and Qsymia for their effects on weight loss.

Full disclosure, the medications we talk about here are not all available in Canada. The information presented here is a combination of research, experience from non-Canadian clinicians, as well as my own experience with some of my patients taking these medications.

What is phentermine, topiramate, and Qsymia?

Phentermine, which goes under many trade names like Adipex or Lomaira, is officially approved for only short-term use by the FDA for weight loss, around 3 months, although it’s frequently taken for longer.

Qsymia is approved for long-term use, like over 6-12 months.

Topiramate, which also goes under many trade names like Topamax or Trokendi, is a medication used officially for treating seizures, epilepsy, and migraines.

Qsymia is actually a combination of both phentermine and topiramate.

So there will be some similarities between Qsymia and the other two. So in this article, I’ll do my best to focus on the differences between all three drugs.

Comparing Phentermine vs Topiramate vs Qsymia

How they work

Phentermine works as an appetite suppressant.

Amphetamine-like stimulants like phentermine, often suppress appetite by stimulating the sympathetic nervous system, also known as the fight-or-flight nervous system.

Normally, this system is activated during times of acute stress, and prepares your body for fight-or-flight by getting your muscles ready to act.

It also shuts down unnecessary systems in your body, like your digestive system. A side effect of this shutdown is a suppression of appetite.

Topiramate also works as an appetite suppressant, although its exact mechanism of action is not really well known.

Qsymia combines the best of both worlds so to speak, by combining the effects of both phentermine and topiramate to create appetite suppression, while minimizing the dose of each to reduce as many side effects as possible.

Side effects

• phentermine – stimulant-related side effects
• topiramate – drowsiness-related side effects
• Qsymia – mostly stimulant-related side effects, although drowsiness-related side effects are also possible

Phentermine is a stimulant, and is commonly associated with insomnia, nervousness, and heart palpitations.

Topiramate is kind of the opposite, and is frequently associated with drowsiness and fatigue.

Phentermine is more associated with cardiovascular side effects like heart palpitations or rapid heart rate, while topiramate is more associated with neurological side effects like dizziness, confusion, mood changes, or muscle coordination issues.

They both have gastrointestinal side effects like nausea or abdominal pain, and may also change the taste of your food, making it taste strange or different.

In Qsymia, the stronger stimulant effect of phentermine is somewhat counteracted by the drowsiness effect of topiramate, although people can still end up potentially experiencing the side effects of taking both phentermine and topiramate.

Regardless, you should be properly screened and regularly assessed by your clinician while on any of these drugs, since they can all potentially cause serious medical issues if taken inappropriately.

All three drugs should not be used during pregnancy.

Approved uses and availability

• topiramate – available in Canada, technically not approved for weight loss
• phentermine – not available in Canada, easy to obtain outside Canada
• Qsymia – not available in Canada, more difficult to obtain in general

Only topiramate is available in Canada at the moment, specifically for the treatment of epilepsy and migraines.

Phentermine and Qsymia are currently not legally available for prescription in Canada. So online pharmacies that claim to sell them to Canadians are not doing so legally or safely.

Outside of Canada, phentermine is used very commonly for weight loss, and topiramate is frequently used off-label for the same purpose.

Costs

1. topiramate – cheapest, generics exist
2. phentermine – cheap, generics exist
3. Qsymia – expensive, no generics available

Generics for phentermine and topiramate exist. Even without insurance, the cost for either is well under 100 dollars per month. With insurance it could be like 10 to 20 dollars per month.

Qsymia is the most expensive, at around a few hundred dollars per month, and is rarely covered by insurance.

Effectiveness

Comparing the weight loss effects of phentermine and topiramate is not easy, since studies rarely compare them head to head, and topiramate is technically not even a drug approved for weight loss.

However, the available research seems to suggest that phentermine is more effective in the short-term, with some people able to lose almost 10% current body weight, but quickly loses efficacy within weeks.

As such, rebound weight gain is much more common upon discontinuing phentermine.

Topiramate is less effective in the short-term, with people less often losing weight, and when they do, it’s usually modest, around 3% current body weight.

However, its weight loss effects are continual and sustained even past a year, suggesting its long-term effects on weight loss are better than phentermine.

Qsymia combines both, and unsurprisingly is more effective than using either drug individually.

In fact, comparison studies with other weight loss drugs like orlistat or Belviq, seem to show that Qsymia is one of the more effective weight loss drugs we have on the market to date.

Summary

They all help to suppress appetite, so if you find yourself overeating and snacking too often, they may be helpful.

Phentermine is best if used short-term, since it’s effects on weight, albeit strong, are not sustained, and you will need to have your diet take over as soon as possible.

Topiramate is not commonly used by itself for weight loss, due to its weaker effects on weight loss, and off-label use, but is frequently used in combination with phentermine.

In fact, a commonly used strategy by some weight loss clinics is to start patients on phentermine, and when phentermine stops working, to then add a slow-release topiramate like Trokendi XR.

This is usually far cheaper for patients to do than starting Qsymia. Kind of like a budget DIY Qsymia.

It won’t be exactly the same as taking Qsymia however, since the proper dosing and release timings for each component in Qsymia is a little different than in the many forms of phentermine and topiramate.

Reportedly however, for many patients, it seems to be good enough.

Qsymia is the most effective, but is the costliest solution.

Also if you do try Qsymia first, and it doesn’t work for you at all, trying phentermine or topiramate individually may not work too well for you either.

Regardless, you’ll need to check with your doctor to see if any of them are appropriate for you to use. And remember that the drugs should not replace the implementation of good dietary and lifestyle habits for losing weight.

Learn more about

• What to Expect with Phentermine
• What to Expect with Qsymia
• 5 Things To Know Before Starting Weight Loss Medication
• Other weight loss medications

See also

• 
  Comparing Weight Loss Drugs in 2024
  Ozempic and Mounjaro are highly effective at treating obesity, however there are many other approved and off-label options that also work.
• 
  Bupropion vs Contrave vs Naltrexone
  Contrave is a synergistic combination of buproprion and naltrexone, which can be replicated to some degree using the generics individually.
• 
  New Obesity Drugs in 2024
  New obesity drugs target more than just GLP-1 and do much more than suppress appetite for a more nuanced approach to weight loss.
• 
  Mounjaro: Who Loses the Most Weight?
  White or Asian younger women who use metformin and have lower sugar and lipid levels tend to experience more weight loss with Mounjaro.
• 
  How does Oforglipron compare?
  Orforglipron, Semaglutide, and Danuglipron are oral GLP-1 agonists that are all similar enough that cost may end up being the winning factor.