What is the drug phentermine hydrochloride used for?

October 9, 2025 •

3 min read

Phentermine is one of the most prescribed weight-loss medications in the United States. This prescription drug is used for short-term weight reduction in people with obesity, but what is the drug phentermine hydrochloride used for specifically, and how does it work to reduce appetite?

Quick Summary

Phentermine hydrochloride is a prescription appetite suppressant for short-term obesity treatment alongside a low-calorie diet and exercise. It stimulates the central nervous system to decrease hunger.

Key Points

Primary Use: Phentermine is a prescription appetite suppressant for short-term weight loss in overweight or obese adults, used in conjunction with a reduced-calorie diet and exercise.
Mechanism of Action: It acts as a central nervous system stimulant, releasing catecholamines like norepinephrine to suppress appetite.
Duration of Treatment: This medication is approved for short-term use, typically for a few weeks, due to the potential for tolerance and dependence.
Controlled Substance: Due to its similarity to amphetamines, phentermine is a Schedule IV controlled substance, necessitating careful prescription and monitoring.
Significant Side Effects: While generally effective, phentermine carries risks of side effects, including increased heart rate, elevated blood pressure, and, in rare cases, serious heart and lung problems.
Contraindications: It is not suitable for individuals with a history of cardiovascular disease, uncontrolled hypertension, hyperthyroidism, glaucoma, or drug abuse.
Alternative Options: For long-term weight management, alternative medications with different mechanisms and side effect profiles, such as GLP-1 agonists, are often considered.

Understanding Phentermine Hydrochloride

Phentermine hydrochloride is a prescription medication primarily used as a short-term adjunct to a weight-reduction program based on exercise, behavioral modification, and caloric restriction. It is intended for patients with obesity (Body Mass Index or BMI of 30 or greater) or those who are overweight (BMI of 27 or greater) with at least one weight-related condition, such as hypertension, diabetes, or high cholesterol. It is classified as a sympathomimetic amine, a type of central nervous system (CNS) stimulant.

How Phentermine Hydrochloride Works

Phentermine's mechanism of action, while not fully understood, involves stimulating the central nervous system. It acts on the hypothalamus region of the brain, which is responsible for regulating appetite. Phentermine stimulates the release of neurotransmitters, primarily norepinephrine, but also dopamine and serotonin, which activates the body's "fight or flight" response, leading to decreased hunger. This suppression of appetite helps patients reduce food intake and achieve a caloric deficit for weight loss.

Primary Medical Use: Weight Loss

Phentermine is prescribed for the short-term treatment of exogenous obesity, typically for a few weeks, due to the potential for tolerance and dependence. Maximum weight loss usually occurs within the first weeks of therapy.

Administration

Phentermine is typically administered orally.

Potential Side Effects of Phentermine Hydrochloride

Phentermine can cause side effects, ranging from common and mild to rare and serious. Common side effects may include dry mouth, headache, insomnia, increased heart rate, and elevated blood pressure. Serious but rare side effects can involve the heart and lungs, such as primary pulmonary hypertension, and require immediate medical attention.

Who Should Not Use Phentermine Hydrochloride (Contraindications)

Phentermine is not suitable for individuals with conditions like cardiovascular disease, uncontrolled hypertension, hyperthyroidism, glaucoma, or a history of drug abuse. It should also not be used during pregnancy or breastfeeding, or within 14 days of taking an MAOI.

Comparison with Other Weight-Loss Medications

Phentermine is a short-term option, differing from newer medications approved for chronic weight management.


Feature	Phentermine	Semaglutide (Wegovy)	Tirzepatide (Zepbound)	Orlistat (Xenical)
Mechanism	Stimulates CNS to suppress appetite via catecholamines.	GLP-1 receptor agonist, slows gastric emptying, and regulates appetite.	GIP and GLP-1 receptor agonist, enhances appetite control and satiety.	Blocks fat absorption in the intestines.
Duration	Short-term (a few weeks), typically up to 12 weeks.	Long-term use.	Long-term use.	Can be used long-term.
Administration	Oral tablet, capsule, or orally disintegrating tablet (ODT).	Weekly subcutaneous injection.	Weekly subcutaneous injection.	Oral capsule, taken with meals.
Side Effects	Dry mouth, increased heart rate, blood pressure changes, insomnia.	Nausea, diarrhea, vomiting, constipation, abdominal pain.	Nausea, vomiting, diarrhea, abdominal pain.	Oily stool, gas, abdominal cramps.
Controlled Status	Schedule IV controlled substance.	Not a controlled substance.	Not a controlled substance.	Not a controlled substance.

Important Considerations and Risks

A medical evaluation is essential before starting phentermine. Patients with certain health histories require close monitoring or alternative treatments. Tolerance to the appetite-suppressing effect can occur, and increasing the dose is not recommended; instead, the medication should be stopped. Abrupt discontinuation after high-dose use can cause withdrawal symptoms. Alcohol should be avoided while taking phentermine.

Conclusion

Phentermine hydrochloride is an effective short-term appetite suppressant for weight loss when combined with diet and exercise in appropriate patients. While it can lead to significant initial weight loss, it is not a long-term solution due to potential serious side effects and dependence. Strict adherence to a healthcare provider's guidance on administration and duration is critical, as is awareness of contraindications and risks. For ongoing weight management, longer-term medication options may be more suitable.

For more information on phentermine, you can consult the MedlinePlus Drug Information.

Frequently Asked Questions

The primary function of phentermine hydrochloride is to act as an appetite suppressant, helping overweight and obese individuals to lose weight in the short term when used alongside a diet and exercise program.

Phentermine helps with weight loss by stimulating the central nervous system, which leads to the release of neurotransmitters like norepinephrine. This increases feelings of fullness and suppresses hunger, helping to reduce calorie intake.

Individuals with a history of cardiovascular disease, uncontrolled high blood pressure, hyperthyroidism, glaucoma, a history of drug abuse, and those who are pregnant or breastfeeding should not take phentermine.

Common side effects include dry mouth, unpleasant taste, increased heart rate, high blood pressure, restlessness, insomnia, dizziness, and gastrointestinal issues like constipation or diarrhea.

No, phentermine is only approved for short-term use, typically a few weeks, because of the potential for developing tolerance and dependence. Its effectiveness tends to decrease over time.

Yes, phentermine is a Schedule IV controlled substance due to its chemical similarity to amphetamines and the potential for abuse and dependence.

Yes, alternatives include longer-term options such as semaglutide (Wegovy), tirzepatide (Zepbound), orlistat (Xenical), and naltrexone/bupropion (Contrave). These medications have different mechanisms of action and may be more suitable for long-term management.