What Is Phentermine?

Phentermine is a prescription appetite suppressant approved by the FDA for the short-term treatment of obesity. It belongs to a class of drugs called sympathomimetic amines and has been available since 1959, making it one of the oldest and most widely prescribed weight-loss medications in the United States. Phentermine is indicated for patients with a BMI of 30 or greater, or 27 or greater with at least one weight-related condition such as high blood pressure, type 2 diabetes, or high cholesterol. It is intended to be used alongside a reduced-calorie diet, exercise, and behavioral changes. Treatment is typically limited to up to 12 weeks, although many physicians prescribe it for longer periods based on clinical judgment.

How Does Phentermine Work?

Phentermine works by stimulating the release of norepinephrine in the brain, particularly in the hypothalamus — the region that controls appetite. This triggers a fight-or-flight response that reduces hunger signals and increases energy expenditure. Phentermine may also modestly increase dopamine and serotonin levels. The combined effect suppresses appetite, making it easier for patients to stick to a calorie-restricted diet. Because Phentermine is a stimulant, it also provides a mild energy boost, which can help patients stay active during weight loss.

What Doses Are Available for Phentermine?

• Phentermine 37.5 mg tablets — most commonly prescribed, taken once daily
• Phentermine 15 mg capsules — lower dose option, taken once daily
• Phentermine 30 mg capsules — once daily dosing
• Phentermine 8 mg tablets (Lomaira) — taken three times daily, 30 minutes before meals
• Adipex-P 37.5 mg tablets and capsules — brand-name equivalent of generic 37.5 mg

How Hard Is It to Find Phentermine in Stock?

Phentermine is generally available at most pharmacies, earning a findability score of 75 out of 100. As a widely prescribed generic, it is stocked by the majority of retail and chain pharmacies. However, because Phentermine is a Schedule IV controlled substance, DEA manufacturing quotas can occasionally limit supply. Some patients have reported intermittent difficulty finding specific strengths (particularly 15 mg capsules) at their local pharmacy since 2022, driven partly by the overall surge in demand for weight-loss medications. Phentermine is not currently listed on the FDA Drug Shortage database. If your pharmacy is out of stock, try calling nearby pharmacies or use Medfinder to locate availability near you.

Who Can Prescribe Phentermine?

Phentermine can be prescribed by any licensed physician or qualified healthcare provider with DEA prescribing authority for Schedule IV substances. The most common prescribers include:

• Primary care physicians and family medicine doctors — the most frequent prescribers
• Internal medicine doctors
• Obesity medicine and bariatric medicine specialists
• Endocrinologists
• Weight management clinics — including telehealth-based weight loss programs

Phentermine is also available through many telehealth platforms, making it convenient to obtain a prescription without an in-person visit in most states.

Is Phentermine a Controlled Substance?

Yes, Phentermine is classified as a Schedule IV controlled substance by the DEA. This means it has a recognized medical use but carries a low-to-moderate potential for physical or psychological dependence. As a controlled substance, Phentermine prescriptions are subject to DEA manufacturing quotas, which can occasionally contribute to supply limitations. Refills may be restricted depending on state law, and some states require in-person visits for initial or ongoing prescriptions. Always obtain Phentermine only with a valid prescription from a licensed provider.

Common Side Effects of Phentermine

• Dry mouth — the most frequently reported side effect
• Insomnia — take the dose early in the morning to minimize sleep disruption
• Increased heart rate and elevated blood pressure
• Constipation or diarrhea
• Restlessness, nervousness, or jitteriness
• Headache and dizziness
• Unpleasant taste

Rare but serious side effects include primary pulmonary hypertension, heart valve problems, chest pain, and shortness of breath. Contact your doctor immediately if you experience any of these symptoms. Phentermine may also cause dependence with prolonged use.

Alternative Medications to Phentermine

• Qsymia (Phentermine/Topiramate ER) — a combination pill that pairs Phentermine with Topiramate for enhanced weight loss; available only through certified pharmacies
• Contrave (Naltrexone/Bupropion) — a non-stimulant, non-controlled oral medication for chronic weight management
• Orlistat (Xenical/Alli) — blocks fat absorption in the gut; Alli is available over the counter at a lower dose
• Wegovy (Semaglutide) — a once-weekly GLP-1 injection approved for weight management; highly effective but expensive
• Saxenda (Liraglutide) — a daily GLP-1 injection for weight loss
• Zepbound (Tirzepatide) — a once-weekly dual GLP-1/GIP injection for obesity; among the most effective options available

Drug Interactions with Phentermine

• MAO inhibitors — do NOT take Phentermine within 14 days of an MAO inhibitor; this combination can cause a dangerous hypertensive crisis
• Other stimulants and sympathomimetic amines — increased risk of cardiovascular side effects
• SSRIs, SNRIs, and triptans — risk of serotonin syndrome when combined with Phentermine
• Antihypertensive medications — Phentermine may reduce the effectiveness of blood pressure drugs
• Insulin and oral diabetes medications — weight loss may change blood sugar levels, requiring dose adjustments
• Alcohol — may worsen dizziness and CNS side effects

Always inform your prescriber of all medications, supplements, and over-the-counter drugs you are taking before starting Phentermine.

Final Thoughts on Phentermine

Phentermine remains one of the most affordable and accessible prescription weight-loss medications on the market. With generic versions widely available for as little as $10–$40 per month, it offers a cost-effective option for patients who qualify. While it is not currently in shortage, occasional spot shortages can occur due to DEA manufacturing quotas and the growing demand for weight-loss treatments. If you're having trouble finding Phentermine at your local pharmacy, Medfinder can help you locate pharmacies with current stock near you. Always use Phentermine under the supervision of a healthcare provider, and discuss the full range of weight-management options to find the best fit for your needs.