When results from 30 trials were analyzed, long-term users of three widely prescribed weight loss drugs lost an average of 6 to 11 pounds over the course of one to four years.
Between 30% and 40% of the participants stopped taking the drugs before the trials ended.
Users of the drugs Xenical, Meridia, and the unapproved drug Acomplia did lose more weight than dieters who took placebos.
They were also more likely to lose 5% to 10% of their body weight -- an amount that is recognized for reducing the severity of obesity-related risks such as diabetes, heart disease, and other weight-related health problems.
But Raj Padwal, MD, of the University of Alberta, says most people who take weight loss drugs are hoping to lose much more weight than the studies suggest they do.
"People tend to be disappointed with the degree of weight loss they achieve with these drugs, even when they know that modest weight loss will improve their health," he says. "People who are desperate to lose weight are usually willing to try drugs, but if they don't see the results they want in two or three months they don't tend to stay on them."
Pros and Cons of Weight Loss Drugs
The trials included in the analysis all lasted one year or more, with most lasting one to two years. One study comparing Xenical to placebo followed patients for four years.
All the study participants were obese, with average weights in the studies ranging between 192 pounds to 231 pounds.
Sixteen of the trials compared Xenical to placebo, 10 did the same with Meridia, and four followed patients taking Acomplia.
Xenical use was associated with a 6-pound average weight loss over the course of the studies, while Meridia users lost an average of 9 pounds, and Acomplia users lost 10 pounds.
Use of Xenical was associated with improvements in total cholesterol, blood pressure, and blood sugar control in patients with diabetes, but 15% to 30% of participants in most of the Xenical studies complained of gastrointestinal problems including fecal urgency and gas with oily discharge.
Use of Meridia was associated with slight increases in blood pressure in seven studies.
While no serious safety issues emerged during the trials, Padwal says there is little information about the safety of using the drugs for many years.
The analysis is published in the latest online issue of the journal BMJ.
"Anti-obesity drugs can help people achieve modest weight loss, but they have to be taken indefinitely or the weight will come back," Padwal says. "Patients and their practitioners have to make a bit of a leap of faith if they plan to continue on these medications for several years."
Obesity Researcher Targets Alli
Padwal says the drugs have a place in the management of obesity as long as patients have realistic expectations about what they can achieve with them.
But in an editorial accompanying the analysis, obesity researcher Gareth Williams, PhD, argues against the use of weight loss drugs without medical supervision.
Williams says the recent introduction of a lower-dose version of Xenical -- sold over-the-counter as Alli in the United States -- is more about marketing than sound medicine.
"Selling anti-obesity drugs over the counter will perpetuate the myth that obesity can be fixed simply by popping a pill and could further undermine the efforts to promote healthy living, which is the only long-term escape from obesity," he writes.
In an interview with WebMD, Williams charges that the main beneficiary of the new over-the-counter pill will be Alli manufacturer GlaxoSmithKline.
"I'm afraid I regard this as a rather cynical money-making enterprise on their part," he says. Taking a weight loss pill without medical supervision is likely to distract from the message that people have to make significant lifestyle changes to achieve meaningful weight loss."
A spokeswoman for GlaxoSmithKline tells WebMD that Alli is intended for use only by people who are willing to make these lifestyle changes.
"Alli is neither a magic pill nor a quick-fix solution, and we have certainly never marketed it that way," says Malesia Dunn. "The way this product has been marketed from day one has been to educate the consumer about the importance of making lifestyle changes."
Acomplia manufacturer Sanofi-Aventis also defended its weight loss drug in a statement issued Thursday.
"Although the article describes the weight loss achieved ... as 'modest,' it does recognize that 'even modest amounts of weight loss (5-10%) are beneficial,' particularly in patients at risk for developing type 2 diabetes," company spokeswoman Julissa Viana notes.
A spokeswoman for Meridia manufacturer Abbott voices a similar sentiment.
"Clinical studies demonstrate that losing and maintaining weight loss of 5%-10% significantly reduces serious health risks of obesity like diabetes and cardiovascular disease," she tells WebMD. "When combined with diet and exercise, Meridia can achieve 5%-10% weight loss."