Bontril (phendimetrazine) is not stronger than phentermine in terms of weight loss results. Clinical reviews comparing the older appetite suppressants in this drug class found that no single one produced significantly more weight loss than the others. However, the DEA does classify phendimetrazine as having a higher abuse potential than phentermine, which is one reason people sometimes assume it’s the more powerful drug.
How They Compare on Weight Loss
Both phendimetrazine and phentermine work the same way: they trigger the release of norepinephrine (and possibly dopamine) from nerve endings in the brain, which suppresses appetite. They belong to the same class of drugs, called sympathomimetics, and have been used for weight management since the 1960s and 1970s.
A 2002 review of clinical trials for all the drugs in this class found that while each one beat placebo, none was significantly superior to the others. A meta-analysis of six phentermine studies (doses of 15 to 30 mg per day, lasting 2 to 24 weeks) found patients lost an average of about 6.3 kg total, with roughly 3.6 kg of that attributable to the drug rather than diet alone. Small 12-week trials of phendimetrazine showed similar weight loss to other drugs in the class. So despite any difference in how “strong” they feel, the clinical outcomes are comparable.
Why Phendimetrazine Has a Higher DEA Schedule
This is where the confusion about strength often comes from. Phendimetrazine is a Schedule III controlled substance, while phentermine is Schedule IV. A higher schedule number means the DEA considers a drug to have greater potential for abuse and dependence. Schedule III drugs carry more prescribing restrictions than Schedule IV drugs.
That classification reflects abuse risk, not effectiveness. Phendimetrazine may produce a slightly more noticeable stimulant sensation in some people, which is part of why it’s rated as having higher abuse potential. But that subjective “kick” doesn’t translate to better appetite suppression or more pounds lost in clinical studies.
Dosing Differences
Phendimetrazine comes in two forms: an immediate-release tablet (35 mg, taken two or three times daily before meals) and an extended-release capsule (105 mg, taken once daily before the morning meal). Phentermine is typically prescribed at 15 to 37.5 mg once daily. The milligram numbers aren’t directly comparable because the drugs have different potencies per milligram, but both are dosed to achieve a similar level of appetite suppression over the course of a day.
Side Effects Are Nearly Identical
Because phendimetrazine and phentermine share a mechanism, their side effects overlap almost completely. The most common ones include palpitations, tremors, restlessness, insomnia, dry mouth, constipation, sweating, and irritability. More serious but rarer effects can include irregular heartbeat, high blood pressure, and dependence with prolonged use.
Neither drug is appropriate for people with significant heart disease, moderate to severe high blood pressure, overactive thyroid, glaucoma, or a history of stimulant abuse. Both carry warnings about the risk of pulmonary hypertension and heart valve problems, particularly with long-term use or when combined with other appetite suppressants.
Both Are Meant for Short-Term Use
The FDA approves phendimetrazine only as a short-term addition to a calorie-restricted diet, defined as “a few weeks.” Its label notes that tolerance to the appetite-suppressing effect typically develops within a few weeks, at which point the drug should be stopped rather than increased. Using appetite suppressants in this class for longer than three months has been linked to a 23-fold increase in the risk of pulmonary hypertension.
Phentermine carries similar short-term guidance. This is a key practical point: neither drug is designed to be a long-term solution. They’re intended as a short boost early in a weight loss program while you build the dietary and activity habits that sustain results after the medication stops.
Availability and Prescribing Trends
The brand-name Bontril Slow-Release has been discontinued in the United States, though generic phendimetrazine remains available. Phentermine, on the other hand, is far more widely prescribed. It’s the most commonly used short-term weight loss medication in the U.S. and is available under several brand names as well as generics.
Phentermine’s lower schedule classification makes it slightly easier for providers to prescribe and for pharmacies to stock. Combined with its larger base of clinical data and comparable effectiveness, phentermine is generally the first choice in this drug class. A provider might try phendimetrazine if phentermine hasn’t worked well or has caused side effects that a patient can’t tolerate, but there’s no clinical reason to consider it a stronger or more effective option.