Short answer: Both are appetite suppressants for short-term use of a few weeks, with a calorie-restricted plan, in people with a BMI of 30 or more.12 Phendimetrazine 35 mg is taken two or three times a day; its label is marked CIII. Diethylpropion 25 mg is taken three times a day; its label gives it as Schedule IV. Neither label gives side-effect rates or compares the two.
| Phendimetrazine | Diethylpropion | |
|---|---|---|
| Controlled substance | CIII, as printed on the label1 | Schedule IV2 |
| Approved for | Short-term add-on for obesity, starting BMI 30 or more1 | Short-term add-on for obesity, starting BMI 30 or more2 |
| Label dose | 35 mg two or three times a day, 1 hour before meals; half a tablet may be enough1 | 25 mg three times a day, 1 hour before meals; a midevening tablet if desired for night hunger2 |
| Maximum | 2 tablets three times a day1 | No higher figure given; the dose is not raised if tolerance develops2 |
| When to stop | Without satisfactory loss in the first 4 weeks, for example at least 4 pounds1 | Without satisfactory loss in the first 4 weeks, for example at least 4 pounds2 |
| Heart valve disease | No case reported with phendimetrazine used alone1 | Very rarely reported with diethylpropion alone; link uncertain2 |
| Missed dose | No rule in the label1 | No rule in the label2 |
| Boxed warning | None1 | None2 |
Both warn of pulmonary hypertension, a rare and often fatal lung disease linked to appetite suppressants, with a 23-fold higher risk after more than 3 months of use in one case-control study. Both say that if tolerance develops the drug is stopped rather than the dose raised, that they are not combined with other appetite suppressants, that they are not used within 14 days of an MAO inhibitor, and that they are not recommended after any appetite suppressant in the prior year.12
Phendimetrazine is not used with symptomatic heart disease, moderate or severe high blood pressure, an overactive thyroid, glaucoma, agitation or a history of drug abuse, and only its yellow tablet contains tartrazine.1 Diethylpropion is not used with pulmonary hypertension or severe high blood pressure, may increase seizures in some people with epilepsy, and is not recommended at age 16 and under.2
Which, if either, fits is for your prescriber. More detail: phendimetrazine side effects, diethylpropion side effects, the phendimetrazine and diethylpropion dosing schedules, the phendimetrazine and diethylpropion guides, and Phendimetrazine vs phentermine: how do the two appetite suppressants differ?.