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Last updated: Oct 28, 2021
5 min read

Phentermine: doses, uses, and side effects

steve silvestro

Medically Reviewed by Steve Silvestro, MD

Written by Ellyn Vohnoutka, BSN, RN


If you have any medical questions or concerns, please talk to your healthcare provider. The articles on Health Guide are underpinned by peer-reviewed research and information drawn from medical societies and governmental agencies. However, they are not a substitute for professional medical advice, diagnosis, or treatment.

Obesity is a significant and growing public health challenge in the United States. The prevalence of obesity has increased more than 75% since 1980. It’s estimated that 51% of U.S. adults will have obesity by 2030 (Bersoux, 2017). 

Having overweight or obesity can cause many health complications, including cardiovascular disease, diabetes, and cancer. In fact, obesity is the second leading preventable cause of death after smoking (Bersoux, 2017). 

Due to these high stakes, researchers have been working for decades to create treatments to help Americans with obesity. One of these is the medication phentermine.

Here’s a look at what phentermine does, how you take it, and the potential side effects.


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What is phentermine?

Phentermine, also known as Lomaira and Adipex-P, was released in 1959 for the short-term treatment of obesity. It was meant to be used in combination with a reduced-calorie diet and exercise program (Johnson, 2021). 

This medication is approved for use by individuals with a starting body mass index (BMI) greater than 30 or greater than 27 for those with an obesity-related health condition, such as (Johnson, 2021; Sombra, 2021):

Phentermine is an effective medication, which has made it the most commonly prescribed medicine for treating obesity in the U. S. In fact, a randomized controlled trial looked at the safety and efficacy of phentermine for 12 weeks in participants with controlled diabetes, high blood pressure, or elevated cholesterol. The researchers found that participants who received phentermine lost more weight than the placebo. They also had lower cholesterol levels. (Kang, 2010).

Phentermine is also frequently found in combination medications with other drugs to help stimulate weight loss. The U.S. Food and Drug Administration (FDA) approved the combination of phentermine and topiramate (Qsymia) in 2012 to treat obesity. Topiramate is an anti-seizure medicine that also works as an appetite suppressant (Johnson, 2021).

How does phentermine work?

Phentermine stimulates the central nervous system. It works in the same manner as amphetamines, by stimulating the release of the neurotransmitters norepinephrine and epinephrine in your brain (Johnson, 2021).

When phentermine stimulates these neurotransmitters, it reduces your appetite, making you feel full sooner. It is often prescribed to be taken alone, but phentermine is also available in combination with other weight loss medications (Sombra, 2021).

The Federal Drug Enforcement Administration (DEA) classifies phentermine as a schedule IV controlled substance, meaning it has the potential for abuse and dependence. This is due to its similarities to amphetamines, but phentermine is considered a lower risk drug than amphetamines (Bersoux, 2017).

Most of the concerns for abuse or addiction with phentermine have not been supported by the medical literature. People who suddenly stop taking phentermine have not been shown to experience withdrawal symptoms (Bersoux, 2017).

How do you take phentermine?

Phentermine is a pill you take by mouth once per day, either before breakfast or up to two hours afterward. It’s given in doses of 15, 30, or 37.5 milligrams (Bersoux, 2017).

One common technique used by some healthcare providers is to prescribe the generic 37.5 mg tablet and instruct the user to take half a tablet (18.75 mg) each morning (Narayanaswami, 2017).

The FDA approves taking phentermine for 12 weeks or less, although it is frequently prescribed off-label for longer than the approved time period (Tirthani, 2021; Narayanaswami, 2017).

The longest trial conducted with phentermine was a 36-week trial that took place in 1968. It looked at continuous and intermittent use of phentermine compared to a placebo. Both ways of taking phentermine were found to be safe and resulted in more significant weight loss than the study participants taking placebo (Sombra, 2021).

What are the side effects of phentermine?

Since phentermine is chemically similar to amphetamines, they share many of the same potential adverse effects. Some common side effects from phentermine include (Narayanaswami, 2017; Bersoux, 2017; National Institute of Diabetes and Digestive and Kidney Diseases, 2012):

  • Heart palpitations
  • Elevated heart rate
  • High blood pressure
  • Nervousness
  • Trouble sleeping
  • Dry mouth
  • Headache
  • Sweating 
  • Constipation
  • Thirst

These effects tend to be worse right when you start taking the medication and decrease with continued use (Bersoux, 2017).

Rare side effects that have been reported with phentermine include (National Institute of Diabetes and Digestive and Kidney Diseases, 2012):

  • Atrial fibrillation (an irregular heartbeat)
  • Sudden psychosis
  • Pulmonary hypertension

Who shouldn’t take phentermine?

Not all medications are suitable for everybody. Your healthcare provider can help you determine if phentermine might be helpful in your weight loss efforts.

People with heart disease should use extreme caution when using phentermine because it can cause increased blood pressure and an elevated heart rate (Bersoux, 2017).

If you have any of the following risk factors or conditions, you should not use phentermine (Bersoux, 2017):

  • Hyperthyroidism (an overactive thyroid)
  • Glaucoma
  • Agitation
  • A history of drug abuse
  • Current pregnancy
  • Breastfeeding
  • Currently using a class of antidepressants called monoamine oxidase inhibitors (MAOIs)

People with a history of psychiatric or cognitive disorders should be cautious when using phentermine. Symptoms such as anxiety, depression, or insomnia can occur. If you have a history of a mood disorder, these symptoms can come back when you use phentermine. Your provider should monitor you carefully for changes in your mood and behavior (Johnson, 2021).

How do you get phentermine?

Phentermine is only available by prescription. If you think you may need some additional help with losing weight on top of diet and exercise, contact your healthcare provider to see if phentermine or another option is right for you. Make sure to tell your provider if you are taking any other medications or supplements to prevent drug interactions.

You and your healthcare professional can decide if phentermine is right for your situation. While many people do lose weight while taking phentermine, it can take several months. Your provider can also help you set realistic expectations for your anticipated results (Johnson, 2021).

Most importantly, your provider can also help you develop a healthy approach to eating and exercise that will be best for you.


  1. Bersoux, S., Byun, T. H., Chaliki, S. S., & Poole, K. G. (2017). Pharmacotherapy for obesity: what you need to know. Cleveland Clinic Journal Of Medicine, 84(12), 951–958. doi: 10.3949/ccjm.84a.16094. Retrieved from https://pubmed.ncbi.nlm.nih.gov/29244650/
  2. Johnson, D. B. & Quick, J. (2021). Topiramate and phentermine. [Updated Jul 29, 2021]. In: StatPearls [Internet]. Retrieved on Oct. 11, 2021 from https://www.ncbi.nlm.nih.gov/books/NBK482165/
  3. Kang, J. G., Park, C. Y., Kang, J. H., Park, Y. W., & Park, S. W. (2010). Randomized controlled trial to investigate the effects of a newly developed formulation of phentermine diffuse-controlled release for obesity. Diabetes, Obesity & Metabolism, 12(10), 876–882. doi10.1111/j.1463-1326.2010.01242.x. Retrieved from https://pubmed.ncbi.nlm.nih.gov/20920040/
  4. Narayanaswami, V. & Dwoskin, L. P. (2017). Obesity: current and potential pharmacotherapeutics and targets. Pharmacology & Therapeutics, 170, 116–147. doi: 10.1016/j.pharmthera.2016.10.015. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5274590/
  5. National Institute of Diabetes and Digestive and Kidney Diseases. (2020). Phentermine. [Updated Jun 4, 2020]. In: LiverTox: Clinical and Research Information on Drug-Induced Liver Injury [Internet]. Retrieved on Oct. 11, 2021 from https://www.ncbi.nlm.nih.gov/books/NBK547916/
  6. Sombra, L. R. S. & Anastasopoulou, C. (2021). Pharmacologic therapy for obesity. [Updated Jul 18, 2021]. In: StatPearls [Internet]. Retrieved on Oct. 11, 2021 from https://www.ncbi.nlm.nih.gov/books/NBK562269/
  7. Tirthani, E. & Quartuccio, M. (2021). Non-dieting approaches to treatment of obesity. [Updated Jun 23, 2021]. In: StatPearls [Internet]. Retrieved on Oct. 11, 2021 from https://www.ncbi.nlm.nih.gov/books/NBK572129/


Dr. Steve Silvestro is a board-certified pediatrician and Manager, Medical Content & Education at Ro.