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Patients taking phentermine long-term to treat obesity don't have to worry about withdrawal when they come off it, researchers found.

Although appetite was temporarily increased the day after stopping phentermine, no other parameters on a withdrawal questionnaire were significantly affected (P=0.084), Ed Hendricks, MD, of the Center for Weight Management in Sacramento, Calif., and colleagues reported at the European Congress on Obesity in Liverpool, England.

"Other than hyperphagia, a symptom reflecting the loss of phentermine therapeutic effect after drug cessation, amphetamine-like withdrawal symptoms did not occur when phentermine was withheld," Hendricks said in a statement. "Abrupt cessation of phentermine after long-term treatment did not induce amphetamine-like withdrawal."

Phentermine, a common diet drug that has been on the U.S. market since 1959, is pharmacologically similar to amphetamine. It's often prescribed long-term to help patients lose weight.

Given its similarity to the stimulant amphetamine, some researchers have suspected that phentermine may also lead to withdrawal once it's stopped.

To test that hypothesis, Hendricks and colleagues assessed 76 patients who'd been treated with phentermine for a median 8.4 years. Mean dose was 54 mg/day.

Patients were given a 10-question Amphetamine Withdrawal Questionnaire modified for phentermine while they were taking the drug, and then again for the next 2 days when the drug was withheld.

Overall, total scores were significantly different across the 3 days, ranging from 3.39 while on-drug to 3.55 on the first day off-drug and 2.99 on the second day (P=0.046).

But only the hyperphagia score was significantly different across groups, rising from 0.77 on-drug to 1.16 the first day off the drug and 1.03 on the second day (P=0.027).

Thus, after hyperphagia score was removed from the total, the remaining scores based on 9 items were not significantly different across days, the researchers reported (P=0.084).

Hendricks and colleagues noted that these scores are far lower than scores achieved on the amphetamine withdrawal questionnaire.

He also added that phentermine is not available in Europe: "Worldwide usage of phentermine has been curtailed because of persistent fears that the drug might induce addiction and that it might induce cardiovascular harm. However, this study suggests fears of causing addiction with long-term phentermine prescribing are exaggerated and present a needless barrier to better care for overweight and obese patients worldwide," he said in a statement.

The drug's structural similarity to amphetamine "has led many to presume that phentermine should have the same adverse effects as crystal methamphetamine," Hendricks said. "No evidence supporting such presumptions is to be found."

"On the contrary," he added, "the evidence strongly suggests phentermine is far safer than is commonly assumed. It is indeed unfortunate that these misconceptions hinder the availability of an effective, affordable medicine for treating obesity."

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