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Key takeaways
Phentermine is a prescription medication used for weight loss in people with a body mass index (BMI) above 30 kg/m2 or 27 kg/m2 if they have certain other health conditions.
Phentermine is an appetite supressant. After taking a dose, it starts to work quickly and reaches the highest levels in the blood within three to four hours. However, weight loss may not be noticeable for a few weeks.
To maximize the results of phentermine, the drug should be taken with a balanced diet and regular exercise.
Phentermine is a prescription medication used with diet, exercise, and behavioral changes for weight loss. It’s typically prescribed for short-term use and works by suppressing appetite and minimizing food cravings. Although phentermine is one of the oldest weight-loss pills on the market, it can be quite effective. But it may take a few weeks to see results.
Read on to learn how long it takes phentermine to work.
RELATED: Where can I get phentermine?
How long does phentermine take to work?
When phentermine was first FDA approved, there wasn’t any long-term data to prove its safety. For this reason, it is only approved for short-term use (12 weeks or less). However, there is growing evidence that it may be safe and effective when taken long-term.
After taking a dose, phentermine starts working to reduce your appetite quickly. It reaches the highest levels in your body in about three to four hours. But it may take a few weeks for you to lose weight. In studies, the rate of weight loss was the highest in the first few weeks of therapy.
What can affect how long it takes phentermine to work?
Phentermine can be taken with or without food. Fortunately, food doesn’t interfere with how phentermine works. Nevertheless, healthcare providers usually recommend you take your dose about two hours after breakfast, as one 30 mg-dose has shown to suppress your appetite for 12 to 14 hours.
Similar to other weight-loss drugs, phentermine should be taken with a reduced-calorie diet and regular exercise. Incorporating these changes while taking phentermine may speed up results. While it may be tempting to take other weight-loss products with phentermine to enhance your results, this isn’t recommended. It’s not known whether it’s safe or effective to take phentermine with other weight-loss aids, including prescribed drugs, over-the-counter products, and herbal supplements.
What is the standard phentermine dosage?
The dosage of phentermine prescribed will vary based on several factors, such as the severity of your condition, other medications you take, and your kidney health. The typical adult dose is 15 mg to 30 mg once daily about two hours after breakfast. It’s best to avoid taking phentermine late in the evening because it can cause insomnia (trouble sleeping). If you have kidney problems, your healthcare provider may prescribe a lower dose.
Phentermine comes in three forms: an oral capsule, oral tablet, and orally disintegrating (dissolvable) tablet (ODT). The oral capsule and ODT are available in strengths of 15 mg, 30 mg, and 37.5 mg, while the oral tablet comes in 8 mg and 37.5 mg.
Who should avoid taking phentermine?
Although phentermine is an effective weight-loss drug, it’s not safe for everyone. Phentermine is a central nervous system stimulant that is chemically similar to amphetamine. It’s classified as a Schedule IV controlled substance by the U.S. Drug Enforcement Administration (DEA) due to its potential for misuse and addiction. As with all medications, phentermine also has several risks and isn’t recommended for people with certain health conditions. These include:
- Cardiovascular disease, such as stroke, heart failure, and uncontrolled high blood pressure
- Hyperthyroidism
- Glaucoma
- Agitation
- History of drug abuse
- Previous allergic reactions to phentermine or similar drugs
- Pregnancy
- Breastfeeding
Phentermine should also not be taken with monoamine oxidase inhibitors (MAOIs), such as Nardil (phenelzine), Emsam (selegiline), and Marplan (isocarboxazid). Taking these drugs together can increase the risk of hypertensive crisis (severely high blood pressure).
Healthcare professionals may also ask you to limit alcohol consumption during phentermine treatment. Mixing alcohol and phentermine can increase the risk of serious side effects, including heart problems.
The bottom line
Phentermine is an effective weight-loss drug that’s approved for use in patients with a body mass index (BMI) of 30 kg/m2 or 27 kg/m2 if they have certain health conditions. The drug should be taken with a healthy diet and regular exercise to maximize its benefits.
Once a dose is taken, phentermine starts to work within three to four hours. However, weight loss is a gradual process, and it may take a few weeks for results to be noticeable. If you do not notice any weight loss after this time, talk to your doctor. They can help determine why you aren’t seeing results and discuss strategies that may make the medication more effective.
- Long-term efficacy and safety of anti-obesity treatment: Where do we stand? Current Obesity Reports (2021)
- Orange book, Food and Drug Administration (FDA)
- Phentermine hydrochloride capsules prescribing information, DailyMed (2024)
- Safety and effectiveness of longer-term phentermine use: Clinical outcomes from an electronic health record cohort, Obesity (Silver Spring) (2019)