Phentermine
An older, stimulant-type appetite suppressant taken by mouth. The label describes it as a short-term add-on to diet and exercise.
Phentermine (known as Adipex-P, Lomaira)
Also searched as: Adipex-P, Lomaira, phentermine hydrochloride.
Educational information. This page does not give dosing instructions or tell you what to take. Ask a prescriber about your own care.
In plain English
Phentermine is the oldest of the current weight-loss drugs and one of the most prescribed. It is a stimulant-type medicine related to amphetamines. It lowers appetite by raising the brain chemical norepinephrine. It does not act on GLP-1.
It is approved only for short-term use, which the label describes in terms of weeks. The label also says its extra weight loss beyond diet alone is modest over the long term. The best-known long-term trial data for any phentermine product is for the Qsymia combination, not phentermine alone.
Because it is a stimulant, it is a Schedule IV controlled substance and is not for people with heart disease, uncontrolled high blood pressure, overactive thyroid, glaucoma, or a history of drug misuse. Pregnancy is a contraindication.
Brand names and products
A generic name identifies the drug itself. A brand name identifies one company's product. One drug can come in several products with different forms and approved uses, so the brand name often tells you more about what a prescription is for.
Sold in the U.S. as Lomaira. A separate page for this brand has not been added to the atlas yet. The forms table below lists its approved use.
Approved and investigational. Phentermine is FDA-approved as a short-term add-on for weight reduction. Longer use, or use with other weight-loss drugs, is outside what the label describes.
How it works
Phentermine is a sympathomimetic amine, in the same broad family as amphetamines. It increases release of norepinephrine in the brain, which reduces appetite and raises alertness and heart rate. The label describes tolerance to the appetite effect as possible.
Forms and approval status
| Form | How it is taken | U.S. status |
|---|---|---|
| Phentermine hydrochloride tablets (Lomaira and other brands) | Daily, as set by the prescriber. Short-term use only, per the label. It should not be combined with other weight-loss drugs. | Approved in the U.S. as a short-term add-on to a reduced-calorie diet, exercise and behavior change for adults with obesity, or overweight plus risk factors such as controlled high blood pressure, diabetes or high cholesterol. Other brands and strengths, including Adipex-P, are also sold; only the Lomaira label was checked here. It is a Schedule IV controlled substance. |
What the trials found
These are group averages from published trials in adults without diabetes unless stated. See how to read trial numbers before comparing.
Low-dose phentermine before weight-loss surgery
Average change in body weight. Every bar uses one scale, 0 to 25 percent.
- People studied
- 53 adults waiting for gastric bypass or sleeve gastrectomy; 45 finished. Average body mass index 48.4; 91% were women
- Length
- 14 weeks
- Compared with
- Placebo
- What was measured
- Average percent total weight loss over 14 weeks
- Analysis approach
- As reported in the abstract. The same trial found average weight loss of 6.2 kg with phentermine vs 1.1 kg with placebo.
- Other results
- Operating time and complications after surgery did not differ between the groups.
- Safety
- Side effects are not summarized in the study abstract. See the official Lomaira label linked on the phentermine page.
- Limits
- Small trial in people about to have surgery, so it does not describe results for the general population. It lasted 14 weeks, which is short. Group averages do not predict one person's result.
Sources: Garcia L 2025, Lomaira (phentermine hydrochloride) tablets: prescribing information
Safety
Based on the FDA-approved label for Lomaira. The official label is the controlling source for warnings. The points below are a summary and may be incomplete.
Boxed warning and label notes
- Phentermine hydrochloride tablets (Lomaira and other brands): No boxed warning appears on the Lomaira label.
Warnings labels describe
- Not for people with cardiovascular disease, uncontrolled high blood pressure, hyperthyroidism, glaucoma, agitation, a history of drug misuse, or during or within 14 days of an MAOI antidepressant. Not for use in pregnancy.
- Related to amphetamines, with a possibility of abuse and dependence. It can impair driving and machinery use.
- Not recommended with other weight-loss drugs, including over-the-counter, herbal and serotonergic products.
- Rare cases of pulmonary hypertension and heart-valve disease have been reported.
Most common in trials
- Fast or pounding heartbeat
- Raised blood pressure
- Restlessness or overstimulation
- Insomnia
- Dry mouth
- Unpleasant taste
- Constipation or diarrhea
- Dizziness or headache
Less common but serious
- Raised blood pressure and heart problems
- Pulmonary hypertension (rare)
- Heart-valve disease (rare)
- Psychosis (listed in the label)
- Dependence
What is still unknown
- How well it works beyond the short term. The label itself calls the added benefit clinically modest over the long term.
- Long-term safety, since it is meant for short-term use.
- Results in a large trial of phentermine alone. The one randomized trial summarized here is small (53 people), lasted 14 weeks and was done in people waiting for weight-loss surgery. The Lomaira label does not report percentage weight loss.
Compiled from public information: FDA-approved labels, manufacturer websites, ClinicalTrials.gov and published journal articles. Sources are listed at the bottom of this page.
Sources
- Lomaira (phentermine hydrochloride) tablets: prescribing information KVK-Tech, Inc., on DailyMed (U.S. National Library of Medicine). FDA-approved label revised 12/2023. Official FDA label on DailyMed
- DailyMed: current FDA drug labels U.S. National Library of Medicine. Regulator
- Utilization of low-dose phentermine for weight loss prior to metabolic and bariatric surgery: a prospective, randomized, and placebo-controlled trial Garcia L, Rivas H, Morton J. Obes Sci Pract. 2025;11(1):e70043. PMID 39830713. Journal article