Phentermine for weight loss: doses, uses, and side effects

Obesity is a significant and growing public health challenge in the United States. In fact, the prevalence of obesity has nearly doubled since 1980. It’s estimated that 51% of U.S. adults will have obesity by 2030. While healthy eating and regular exercise are key to weight loss, some medications–like phentermine–can help you lose weight and keep weight off.

Combined with a healthy diet and exercise, phentermine can act as an appetite suppressant to promote and maintain weight loss. However, since the medication is a stimulant, it should be used with caution. 

Continue reading to learn more about phentermine for weight loss, doses, and potential side effects.

Phentermine, also known as Lomaira and Adipex-P, is meant to be used short-term, and in combination with a reduced-calorie diet and exercise program. 

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:

• Uncontrolled high blood pressure

• Diabetes

• Elevated cholesterol levels

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.

Phentermine is effective for promoting weight loss. In fact, a randomized controlled trial over 12 weeks looked at the safety and efficacy of phentermine in participants with controlled diabetes, high blood pressure, or elevated cholesterol. The trial found that participants who received phentermine lost more weight than the placebo and had lower cholesterol levels.

Some research has shown that those who take a daily dose of 15 mg may see weight loss of up to 7% of their initial body weight after six months.

While phentermine is only FDA-approved for short-term use (12 weeks), many healthcare providers prescribe the medication off-label for more long-term use. Or, your healthcare provider may prescribe phentermine intermittently, which means you take a break from the medication before returning to it. 

Unfortunately, not everyone will have success using phentermine. If you haven’t seen adequate results after 3 months of use, your healthcare provider may want to explore other treatment options.

Phentermine works as an appetite suppressant. The medication is a stimulant and works in the same way as amphetamines–by stimulating the release of the neurotransmitters norepinephrine and epinephrine (the hormones behind your “flight or fight” response) in your brain.

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

Phentermine should be used with caution. 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.

Most of the concerns for abuse or addiction with phentermine have not been supported by the research. People who suddenly stop taking phentermine are not known to experience withdrawal symptoms.

Phentermine is a pill you take by mouth once daily, either before breakfast or up to two hours afterward. As mentioned, it is only FDA-approved for short-term use (12 weeks), but some healthcare providers may prescribe it off-label for long-term use. 

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 a placebo.

Phentermine is available in doses of 15, 30, or 37.5 milligrams. 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.

Since phentermine is chemically similar to amphetamines, they share many of the same potential side effects. Some common side effects from phentermine include:

• 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.

Rare side effects that may occur with phentermine include:

• Atrial fibrillation (an irregular heartbeat)

• Sudden psychosis

• Pulmonary hypertension

The combination of phentermine and topiramate may help reduce binge eating in people with binge eating disorders (BED) and bulimia nervosa.

One small, preliminary 12-week study of people with binge eating disorders who were overweight found that participants who took both phentermine and topiramate had reductions in binge-eating episodes.

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.

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

• 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 including 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.

Phentermine for weight loss is only available by prescription. If you think you may need some additional help with losing weight on top of diet and exercise, make an appointment with 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.

While many people do lose weight while taking phentermine, it can take several months. Your provider can help you set realistic expectations for your anticipated results.

If you’re struggling with weight loss, you’re not alone. Make an appointment to create a treatment plan that will be safe and effective for you.

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Every article on Health Guide goes through rigorous fact-checking by our team of medical reviewers. Our reviewers are trained medical professionals who ensure each article contains the most up-to-date information, and that medical details have been correctly interpreted by the writer.

Current version

July 01, 2024

Written by

Ellyn Vohnoutka, BSN, RN

Fact checked by

Felix Gussone, MD

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About the medical reviewer

Felix Gussone, MD, Ro

Felix Gussone is a physician, health journalist and a Manager, Medical Content & Education at Ro.