Phentermine is an FDA-approved appetite-suppressant pill that produces 5 to 8 percent body weight loss over 12 weeks. It works as a sympathomimetic, releasing norepinephrine in the brain to reduce hunger. Side effects include dry mouth, insomnia, palpitations, and modest blood pressure increases. Phentermine is a Schedule IV controlled substance and the most commonly prescribed anti-obesity drug in the US.
What Phentermine Is
Phentermine is a sympathomimetic amine — chemically similar to amphetamine but with much less abuse potential. It was approved by the FDA in 1959 for short-term weight management. Common brand names include Adipex-P (15, 30, and 37.5 mg), Lomaira (8 mg three times daily, the lowest dose), and Suprenza (orally disintegrating tablets). Generic phentermine has been available for decades and is inexpensive.
Phentermine is the most widely prescribed anti-obesity medication in the United States today. Despite the FDA label limiting use to 12 weeks (a holdover from older labeling conventions), obesity specialists routinely prescribe it long-term off-label with monitoring.
Who Qualifies
Phentermine is FDA approved for short-term (a few weeks) use as part of a weight-reduction regimen including exercise, behavior modification, and caloric restriction in adults with:
- BMI of 30 kg/m² or higher, or
- BMI of 27 kg/m² or higher with at least one weight-related risk factor (hypertension, type 2 diabetes, dyslipidemia)
It is not approved for use under age 17.
How It Works
Phentermine acts primarily in the hypothalamus by stimulating norepinephrine release. Norepinephrine activation in appetite centers suppresses hunger. Dopaminergic effects may also contribute. Compared with amphetamine, phentermine has much weaker effects on dopamine release and serotonin, which explains its lower abuse potential.
The effect on appetite is felt within hours of the first dose and usually plateaus over several days. Most patients describe reduced hunger between meals and earlier fullness during meals.
Doses and Forms
| Brand / Form | Strength | Typical Dosing |
|---|---|---|
| Adipex-P (tablets and capsules) | 37.5 mg | 1 tablet daily before breakfast or 1 to 2 hours after |
| Adipex-P (tablets) | 37.5 mg, scored | Can split — half tablet twice daily |
| Generic phentermine | 15 mg, 30 mg, 37.5 mg | Once daily in the morning |
| Lomaira | 8 mg | Three times daily, 30 minutes before meals |
| Suprenza (ODT) | 15, 30, 37.5 mg | Once daily, dissolved on tongue |
Lower doses (8 to 15 mg) often produce most of the appetite-suppressing benefit with less stimulation and fewer cardiovascular side effects. Many obesity specialists start at 15 mg or 8 mg twice daily rather than the 37.5 mg labeled standard.
Expected Weight Loss
Older 12-week trials and modern observational data converge on similar numbers.
| Setting | Average Weight Loss |
|---|---|
| Phentermine 37.5 mg, 12 weeks | ~6-8% body weight |
| Phentermine 15 mg, 12 weeks | ~5-7% |
| Phentermine 8 mg TID (Lomaira), 12 weeks | ~5% |
| Placebo, 12 weeks | ~1-3% |
| Phentermine long-term (off-label, observational) | ~7-9% sustained |
The bulk of weight loss usually occurs in the first 6 to 8 weeks. A common pattern is rapid early loss followed by a plateau; the FDA label’s 12-week recommendation reflects this plateau historically. Continued use beyond 12 weeks typically maintains weight loss rather than producing further large reductions.
Common Side Effects
- Dry mouth — most common
- Insomnia — minimize by taking in the morning
- Palpitations or rapid heart rate
- Anxiety, jitteriness, irritability
- Constipation
- Headache
- Dizziness
- Modest blood pressure rise (typically 2 to 5 mmHg systolic)
- Modest heart rate rise (typically 2 to 5 bpm)
- Decreased libido
- Tremor
Serious Risks
- Cardiovascular events — rare but documented; risk is higher in patients with preexisting cardiovascular disease
- Pulmonary hypertension — a concern carried over from the fenfluramine/phen-fen era, though phentermine alone has not been clearly linked
- Valvular heart disease — same fen-phen-era concern, not associated with phentermine alone
- Substance use risk — particularly in patients with prior stimulant or amphetamine use disorder
- Serotonin syndrome — when combined with SSRIs, SNRIs, triptans, tramadol, or MAOIs
- Pregnancy exposure — phentermine is not recommended in pregnancy; weight loss during pregnancy is generally not advised
Contraindications
- Cardiovascular disease (CAD, stroke, arrhythmia, congestive heart failure, uncontrolled hypertension)
- Pulmonary hypertension
- Hyperthyroidism
- Glaucoma
- Agitated states or history of substance use disorder
- MAOI use within 14 days
- Pregnancy and breastfeeding
- Hypersensitivity to sympathomimetic amines
How Phentermine Compares
| Option | Class | Avg Weight Loss | Cost (generic) |
|---|---|---|---|
| Phentermine | Sympathomimetic | 5-8% (12 wk) | ~$10-30/mo |
| Qsymia | Phentermine + topiramate | 8-10% (1 yr) | ~$98-200/mo |
| Contrave | Naltrexone + bupropion | 5-9% (1 yr) | ~$99/mo (DTC) |
| Orlistat (Alli OTC) | Lipase inhibitor | 3-4% (1 yr) | ~$50/mo |
| Plenity | Hydrogel device | ~6% (6 mo) | ~$98/mo |
| Wegovy / Zepbound | GLP-1 / GIP-GLP-1 | 15-21% (1 yr) | ~$1,000-1,350/mo |
Phentermine offers the best cost-per-pound-lost ratio of any FDA-approved anti-obesity medication, which is why it remains the most commonly prescribed despite older approval and a short-term label.
Phentermine and Prediabetes
The Diabetes Prevention Program (DPP) demonstrated that intensive lifestyle change or metformin can prevent or delay progression from prediabetes to type 2 diabetes. Although phentermine was not a DPP-randomized arm, it was used in some lifestyle protocols. Weight loss in the 5 to 10 percent range — achievable with phentermine for many patients — reliably improves insulin sensitivity, lowers fasting glucose, and may lower A1C. Patients with prediabetes who are not candidates for or cannot access GLP-1 medications often consider phentermine plus lifestyle change.
Practical Considerations
- Take in the morning — evening doses cause insomnia.
- Avoid afternoon caffeine to reduce jitteriness.
- Drink plenty of water for dry mouth and constipation.
- Check blood pressure at 2 to 4 weeks after starting and periodically thereafter.
- Combine with diet and exercise — medication alone underperforms its trial numbers.
- Plan for plateau by week 8 to 12 — adjust expectations rather than escalating dose.
- Discuss whether long-term use makes sense for your situation; the 12-week label is not a hard rule in modern obesity practice.
Related Reading
For broader context, see our anti-obesity medications overview, our guide to Qsymia, and our pillar on treatment options. Also see our explanation of A1C levels.
The Bottom Line
Phentermine is a once-daily oral appetite suppressant that produces 5 to 8 percent body weight loss over 12 weeks. It is the most commonly prescribed anti-obesity medication in the US, largely because it is inexpensive, reliable in the short term, and effective at low doses. Common side effects are dry mouth, insomnia, palpitations, and modest blood pressure rise. Contraindications include cardiovascular disease, uncontrolled hypertension, hyperthyroidism, and substance use history. Phentermine is a Schedule IV controlled substance. Talk to your doctor about whether phentermine fits your medical history and goals, and combine it with sustained lifestyle change for the best long-term results.
