Adipex-P is a brand name for phentermine. The two contain the exact same active ingredient, phentermine hydrochloride, at the same strength: 37.5 mg per tablet or capsule (equivalent to 30 mg of phentermine base). If your pharmacy switches you from one to the other, you are getting the same medication.
Brand Name vs. Generic
The relationship between Adipex-P and phentermine is the same as Tylenol and acetaminophen, or Advil and ibuprofen. Adipex-P is the branded product; generic phentermine is the unbranded version. Both must meet the same FDA standards for the active drug’s purity, potency, and how it’s absorbed by your body.
Where they can differ is in the inactive ingredients: the fillers, binders, coatings, and dyes that hold a pill together and give it its shape and color. These differences are cosmetic for most people, but if you have a known sensitivity or allergy to a specific dye or filler, it’s worth comparing the ingredient lists on the packaging. The drug itself is identical.
What Phentermine Does
Phentermine is a prescription appetite suppressant classified as a sympathomimetic amine, meaning it mimics the activity of your body’s “fight or flight” chemical messengers. It triggers the release of norepinephrine in the brain’s appetite-control center, which creates a sense of fullness and reduces hunger signals. It also nudges levels of dopamine and serotonin upward, both of which play a role in how satisfied you feel after eating.
Beyond curbing appetite, the bump in norepinephrine slightly increases your resting energy expenditure, so you burn a few more calories at rest than you otherwise would. The appetite suppression is the main driver of weight loss, though. Phentermine is not a metabolism booster in any dramatic sense.
Who It’s Prescribed For
Phentermine is approved for short-term weight management in adults with a BMI of 30 or higher, or a BMI of 27 or higher if you also have a weight-related condition like high blood pressure, type 2 diabetes, or high cholesterol. It’s meant to be used alongside a reduced-calorie diet and increased physical activity, not as a standalone fix.
The FDA label specifies use for “a few weeks,” generally interpreted as up to 12 weeks. Some providers prescribe it for longer stretches using an on-off pattern, but the original approval was for short-term use. Because phentermine is chemically related to amphetamines, it carries a Schedule IV controlled substance classification, which means prescriptions are tracked and refills are limited.
Common Side Effects
Most side effects stem from the same stimulant activity that suppresses your appetite. The ones people report most often include dry mouth, difficulty sleeping (especially if taken later in the day), a jittery or restless feeling, increased heart rate, and constipation. Some people also notice elevated blood pressure.
These effects are the same whether you take Adipex-P or generic phentermine, because the active drug is identical. Taking your dose early in the morning, typically before breakfast or one to two hours after, helps minimize sleep disruption. Side effects tend to be strongest in the first week or two and often ease as your body adjusts.
Cost Differences
The most practical difference between Adipex-P and generic phentermine is price. Generic phentermine is widely available and relatively inexpensive, often costing significantly less than the brand-name version for a 30-day supply. Because the drug has been on the market for decades and multiple manufacturers produce generic versions, competition keeps the price low. If your provider writes a prescription for “Adipex-P” specifically, ask whether a generic substitution is an option. In nearly every case, your pharmacist can fill it with generic phentermine unless the prescription explicitly requires the brand.
Other Brand Names for Phentermine
Adipex-P is the most recognized brand, but it’s not the only one. Lomaira is a lower-dose brand containing 8 mg of phentermine hydrochloride, designed to be taken three times a day before meals rather than as a single daily dose. Phentermine is also one half of the combination medication Qsymia, which pairs it with topiramate for a longer-term weight loss option. If you see any of these names, the phentermine component works the same way it does in Adipex-P.