Adipex-P is a brand-name prescription weight loss medication containing phentermine hydrochloride, one of the oldest and most widely prescribed appetite suppressants in the United States. It comes in 37.5 mg tablets and capsules and is approved for short-term use alongside a reduced-calorie diet and exercise in adults with obesity or weight-related health conditions.
Who Adipex-P Is Prescribed For
Adipex-P is FDA-approved for adults with a BMI of 30 or greater (the clinical threshold for obesity). It can also be prescribed at a lower BMI of 27 or greater if you have at least one weight-related condition such as high blood pressure, type 2 diabetes, or high cholesterol. In both cases, it’s meant to supplement lifestyle changes like diet and physical activity, not replace them.
Phentermine is also one of the active ingredients in Qsymia, a combination medication approved for long-term weight management in both adults and adolescents 12 and older. Adipex-P itself, however, is approved only for short-term use in adults.
How It Works
Phentermine is chemically related to amphetamines, though it’s milder. It works by triggering the release of norepinephrine in the part of your brain that regulates hunger. This creates a signal that suppresses appetite and reduces the urge to eat. To a lesser extent, it also increases levels of dopamine and serotonin, brain chemicals involved in mood and feelings of fullness.
Beyond appetite suppression, phentermine activates your sympathetic nervous system, the same system responsible for your “fight or flight” response. This slightly increases your resting energy expenditure, meaning your body burns a bit more energy even at rest. The combined effect of eating less and burning slightly more is what drives weight loss while taking the medication.
How Adipex-P Is Taken
The standard dose is one 37.5 mg tablet or capsule per day, taken before breakfast or one to two hours after breakfast. Some people do well on a lower dose of 18.75 mg (half a tablet), taken once daily or split into two doses throughout the day. Your prescriber will adjust the dose based on how you respond.
Timing matters. Adipex-P is a stimulant, and taking it too late in the day can cause insomnia. The FDA label specifically warns against evening doses for this reason. Most people take it first thing in the morning to get appetite control during waking hours without disrupting sleep.
Controlled Substance Classification
Phentermine is a Schedule IV controlled substance under federal law. That places it in the same category as medications like certain sleep aids and anti-anxiety drugs, meaning it has a recognized potential for dependence, though lower than drugs in Schedules I through III. Because of this classification, prescriptions are typically written for limited quantities at a time, and your prescriber will monitor you closely throughout treatment.
Common Side Effects
Because Adipex-P stimulates your nervous system, most of its side effects reflect that stimulant activity. The most frequently reported include:
- Dry mouth, which is very common and often persistent throughout treatment
- Insomnia, especially if the medication is taken later in the day
- Increased heart rate and elevated blood pressure
- Restlessness or jitteriness, similar to consuming too much caffeine
- Constipation
These side effects tend to be most noticeable in the first week or two as your body adjusts. Some people find the stimulant effects settle down over time, while others remain sensitive to them throughout the course of treatment. If side effects are bothersome, a lower dose (18.75 mg) may help.
Who Should Not Take It
Adipex-P is not safe for everyone. Several medical conditions make it off-limits entirely:
- Heart disease, including coronary artery disease, structural heart abnormalities, serious arrhythmias, or uncompensated heart failure
- Uncontrolled or severe high blood pressure
- Overactive thyroid (hyperthyroidism)
- Narrow-angle glaucoma, because phentermine can raise pressure inside the eye
- Significant anxiety or agitation, which the medication can worsen
- History of drug abuse, given phentermine’s stimulant properties and dependence potential
The cardiovascular restrictions are the most important to understand. Phentermine increases heart rate and blood pressure, and in someone with an existing heart condition, those changes can be dangerous. Your prescriber will typically check your blood pressure and heart health before starting treatment and continue monitoring throughout.
Drug Interactions to Know About
Adipex-P should not be combined with other stimulant-type weight loss medications. Stacking sympathomimetic drugs raises the risk of dangerously high blood pressure and abnormal heart rhythms. It should also not be used within 14 days of taking MAO inhibitors, a type of antidepressant, due to the risk of a hypertensive crisis.
Tricyclic antidepressants can amplify phentermine’s effects in the brain, potentially intensifying both its intended actions and side effects. If you take any antidepressant, your prescriber needs to know before starting Adipex-P so they can weigh the risks and adjust dosing or monitoring accordingly.
What to Expect From Treatment
Adipex-P is designed as a short-term tool. The FDA label describes it as an adjunct to lifestyle changes for a limited period, typically a few weeks to a few months. It is not approved for indefinite use, and its appetite-suppressing effects may diminish over time as your body adapts to the medication.
Most of the weight loss happens because you simply feel less hungry and eat fewer calories. The degree of weight loss varies significantly from person to person and depends heavily on how much you change your eating and exercise habits alongside the medication. People who treat Adipex-P as a kickstart while building lasting habits tend to have better outcomes than those who rely on the medication alone. When you stop taking it, appetite typically returns to its previous level, which is why the behavioral changes you make during treatment matter more than the drug itself for long-term results.