How Much Weight Can You Expect to Lose Using Phentermine and Topiramate?

How Much Weight Can You Expect to Lose Using Phentermine and Topiramate?

How Much Weight Can You Expect to Lose Using Phentermine and Topiramate?
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Phentermine and topiramate (Qsymia) is one of several drugs approved for weight loss in the United States, the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) says.

Phentermine is an appetite suppressant, while topiramate is a drug typically used to treat seizures and migraine. It may make you less hungry or feel satiated sooner, but how much weight does it typically help people lose?

How Phentermine and Topiramate Enhances Weight Loss

According to research, the exact mechanism of action of these two drugs used together isn’t understood.

One theory is that when taken together, they alter how neurotransmitters work in your body. Phentermine enhances the effects of norepinephrine, while topiramate acts on certain gamma-aminobutyric acid (GABA) receptors.

By taking them together, they’re able to suppress appetite better than either drug alone, leading to a greater decrease in calorie intake.

The U.S. Food and Drug Administration (FDA) says the medication is indicated for weight loss when used as an add-on to a reduced-calorie diet and increased physical activity. Phentermine and topiramate is FDA approved for long-term use.

If you’re considering phentermine and topiramate versus dextroamphetamine and amphetamine (Adderall) for weight loss, be aware that phentermine and topiramate is the only one that’s FDA approved for weight loss, the NIDDK says.

According to the University of California in San Francisco, stimulant drugs like dextroamphetamine and amphetamine are no longer prescribed for weight loss due to their side effects and addiction potential. Side effects include anxiety, irritability, heart issues, and high blood pressure.

Monthly Weight Loss

How much weight you will lose when taking phentermine and topiramate varies from person to person and depends on a number of wide-ranging factors, research says, such as:

  • Age
  • Sex
  • Race
  • Genetics
  • Baseline weight
  • Body composition
  • Metabolic rate
  • Co-occurring medical conditions
  • Other medications you may take
  • Diet quality
  • Physical activity
  • Sleep
  • Hydration
  • Psychological factors
  • Stress

You must take the medication as prescribed to get any potential weight loss benefits. That said, existing research studies can give you an idea of what to expect from this medication.

Clinical Trials for FDA Approval

In the product literature for Qsymia, the brand name version for extended-release phentermine and topiramate, the FDA discusses two year-long clinical trials that included about 2,300 people total.

Both studies combined phentermine and topiramate with a well-balanced, reduced-calorie diet. Participants were also offered nutritional and lifestyle counseling.

One trial enrolled people with obesity and excluded those who also had type 2 diabetes. The second study included people who were overweight and those with obesity as well as people with type 2 diabetes and other metabolic conditions, such as high blood pressure.

Only 7 percent of participants overall were 65 to 69 years old. No one over 70 participated, so it’s not known whether older people might get the same results from this drug.

In study 1, the average weight loss at one year was about:

  • 2.1 percent of baseline body weight in the placebo group
  • 6.7 percent of baseline body weight in the lower-dose group (3.75 milligrams [mg] phentermine/23 mg topiramate)
  • 14.4 percent of baseline body weight in the higher-dose group (15 mg phentermine/92 mg topiramate)

In study 2, the average weight loss at one year was about:

  • 1.2 percent of baseline body weight in the placebo group
  • 7.8 percent in the lower-dose group (7.5 mg phentermine/46 mg topiramate)
  • 9.8 percent of baseline body weight in the higher-dose group (15 mg phentermine /92 mg topiramate)

At one year, a greater proportion of participants in both trials lost 5 to 10 percent of their body weight compared with those in the placebo groups.

In study 1, a significant proportion of participants lost 5 percent or more of their body weight: 45 percent in the lower-dose group and 67 percent in the higher-dose group.

Among study 2 participants, an even higher percentage of participants lost 5 percent or more of their body weight. This study administered a higher dose to their lower-dose group than in study 1, which increased the number of people able to lose at least 5 percent of their body weight.

In study 2, 62 percent in the lower-dose group and 70 percent in the higher-dose group lost 5 percent or more of their body weight. In the placebo groups, 17 and 21 percent lost at least 5 percent of their body weight in study 1 and study 2, respectively.

In study 1, a smaller proportion of participants lost 10 percent or more of their body weight: 19 percent in the lower-dose group and 47 percent in the higher-dose group.

Among study 2 participants, an even higher percentage of participants lost 10 percent or more of their body weight: 37 percent in the lower-dose group and 48 percent in the higher-dose group.

In the placebo groups, 7 percent lost at least 10 percent of their body weight in both study 1 and study 2.

Forty percent of the participants in study 1 and 31 percent of participants in study 2 dropped out before the studies’ completion, however. And because of how these trials presented their data, it’s not possible to determine how much the average monthly weight loss was.

According to Cleveland Clinic, you should aim to lose 1 to 2 pounds per week. Taking drastic steps to lose more than that is typically a bad idea because it is not sustainable and makes it more likely you'll regain any lost weight.

Side Effects, Contraindications, and Drug Interactions

Like all prescription medications, phentermine and topiramate has potential side effects, contraindications, and drug interactions, according to the FDA.

People who are pregnant are warned against taking this drug, as it can cause congenital disabilities, such as cleft lip or palate. If you’re able to become pregnant, you’ll need to use reliable birth control while taking phentermine and topiramate, the manufacturer says.

Common side effects include:

  • Numbness or tingling in the arms, hands, feet, or face
  • Dizziness
  • Changes in the way food tastes or loss of taste
  • Insomnia
  • Constipation
  • Dry mouth

Less often, it can cause other side effects such as:

  • Increases in blood pressure or heart rate
  • Mood changes
  • Trouble sleeping
  • Cognitive problems
  • Speech difficulties
  • Decreases in kidney function

Rare side effects include suicidal thoughts and serious eye problems that may lead to vision loss.

Low blood sugar in people taking type 2 diabetes medications can occur. It can cause low blood pressure in people who take blood pressure medications. It can also increase the chance of kidney stones or cause physical dependence. Experts recommend tapering the drug rather than stopping abruptly.

In addition to pregnancy, you should not take phentermine and topiramate if you have glaucoma, hyperthyroidism, or have taken monoamine oxidase inhibitors (MAOIs) in the last two weeks.

Phentermine and topiramate interacts with several types of medications, including:

  • Birth control pills
  • Central nervous system depressants, such as benzodiazepines, muscle relaxers, opioids, and some sleep medications
  • Diuretics that affect potassium

It can also interact with alcohol.

Talk to your doctor if you take phentermine and topiramate and experience side effects.

EDITORIAL SOURCES
Everyday Health follows strict sourcing guidelines to ensure the accuracy of its content, outlined in our editorial policy. We use only trustworthy sources, including peer-reviewed studies, board-certified medical experts, patients with lived experience, and information from top institutions.

Adam Gilden, MD, MSCE

Medical Reviewer

Adam Gilden, MD, MSCE, is an associate director of the Obesity Medicine Fellowship at University of Colorado School of Medicine and associate director of the Colorado University Medicine Weight Management and Wellness Clinic in Aurora. Dr. Gilden works in a multidisciplinary academic center with other physicians, nurse practitioners, registered dietitians, and a psychologist, and collaborates closely with bariatric surgeons.

Gilden is very involved in education in obesity medicine, lecturing in one of the obesity medicine board review courses and serving as the lead author on the Annals of Internal Medicine article "In the Clinic" on obesity.

He lives in Denver, where he enjoys spending time with family, and playing tennis.

Beth Spicer

Author

Beth Spicer is a statistician in the Research Triangle area of North Carolina. Her professional experience includes 20 years working in social, behavioral and medical research for Duke University, GlaxoSmithKline, Family Health International and other area research institutions. She holds a B.S. in mathematics from University of North Carolina-Chapel Hill, and a M.B.A. from The College of William & Mary.