Phentermine: what it treats, how to take it, side effects
Last updated September 3, 2026.
Phentermine (brand name Adipex-P) is the oldest and most prescribed weight-loss drug in America, a stimulant appetite suppressant approved since 1959. It is a Schedule IV controlled substance, indicated only as a short-term adjunct of a few weeks, inside a full program of diet, exercise, and behavior change.
What does it treat?
Exogenous obesity: weight driven by intake, not by a medical cause, in patients with a starting BMI of 30 or higher, or 27 or higher with risk factors. It suppresses appetite through the same norepinephrine pathways as its amphetamine cousins, buying a window where dieting feels less like fighting your own brain.
How do you take it?
One 37.5 mg tablet daily before breakfast or 1 to 2 hours after it; some patients do better on half a tablet (18.75 mg), sometimes twice daily. Morning dosing is deliberate, since taking it late causes insomnia. The labeled course is a few weeks; tolerance to the appetite effect builds within weeks, and when it stops working, the answer per the label is to stop, not to raise the dose. It is the entry half of the combination Qsymia, where topiramate stretches the effect into months. Missed dose: take it when remembered unless the next dose is near; never double up.
Side effects to know
- Common: dry mouth, insomnia, jitteriness, restlessness, faster heart rate, elevated blood pressure, and constipation, the stimulant signature.
- Serious: pulmonary hypertension and valvular heart disease (the reason the fen-phen era ended; phentermine alone carries the warning), dangerous blood-pressure elevation, and abuse or dependence, which is why it is scheduled and why a drug-abuse history weighs against it.
Who should not take it?
Not with a history of cardiovascular disease (coronary disease, stroke, arrhythmias, heart failure, uncontrolled hypertension), hyperthyroidism, glaucoma, agitated states, a history of drug abuse, pregnancy or breastfeeding, or within 14 days of MAO inhibitors. MedlinePlus advises against routine use over age 65. Alcohol amplifies side effects. Diabetics may need insulin adjusted as intake drops.
When is it an emergency?
Call 911 for chest pain, severe shortness of breath, fainting, or a pounding irregular heartbeat, and for signs of dangerously high pressure like severe headache with blurred vision. New exercise intolerance or ankle swelling after weeks on it (possible pulmonary hypertension) gets same-week evaluation, not a wait. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper.
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Common questions
Is phentermine just legal speed?
Chemically it is a sympathomimetic amine in the amphetamine family, which is why it is Schedule IV. At the prescribed 37.5 mg it suppresses appetite more than it produces euphoria, and six decades of use define its risks well, but the resemblance is real: insomnia, jitters, elevated pulse and pressure, and genuine abuse potential.
Why is it only approved for a few weeks?
Because the trials supporting it were short, and tolerance to the appetite effect develops within weeks as receptors adapt. The label is explicit that when it stops working, the dose should not be escalated; the drug should be stopped. Longer real-world use happens off-label under supervision, but that is the official shape.
Wasn't this the phen in fen-phen?
Yes. The 1990s disaster came from combining it with fenfluramine, which caused valvular heart disease and pulmonary hypertension; fenfluramine was withdrawn in 1997. Phentermine alone was never shown to cause that damage, but the label still carries the class warning, and new exercise intolerance or ankle swelling on it deserves evaluation.
Can I get phentermine from an online pharmacy or telehealth?
As a Schedule IV controlled substance, a legitimate first prescription requires a real clinical evaluation, and telehealth rules for controlled substances apply; the details shift with regulation, so the honest answer is: telehealth can evaluate and route appropriately, and no-question online pharmacies shipping it are operating outside the law and outside safety.
What happens when the few weeks are over?
Appetite returns to baseline, and whatever habits you built carry the result. That is the honest design: phentermine buys a window, and the weight trajectory after it depends on the diet, activity, and sometimes a transition to a longer-acting medication like Qsymia (which contains phentermine) or a GLP-1, under your doctor's plan.
Does it interact with my antidepressant?
MAO inhibitors are a hard no within 14 days. With SSRIs and other antidepressants the issue is additive stimulation and blood pressure, and with any psychiatric medication your prescriber should see the whole list. Also flag it to your dentist and surgeon before anything involving adrenaline-containing anesthesia.