Drug makers would like patients to believe that their choice of an antidepressant makes a big difference in how well they'll pull out of the blues.

But a new study designed to simulate everyday medical practice shows that, on average, three of the most popular mood drugs perform equally well in easing adult depression.

Such parity has important implications for health care financing, experts say, because generic versions of these medications may be much less expensive than their branded siblings.

The study focused on 573 adults treated for depression at more than three dozen primary care sites in the United States. Of those, 189 were initially given paroxetine, 193 took fluoxetine, and 191 received sertraline, but they were allowed to switch if they had bad reactions or didn't respond to the drugs.

The three pills are called selective serotonin reuptake inhibitors (SSRIs), because they lift mood by boosting brain levels of the messenger molecule serotonin. Paroxetine is sold as Paxil by GlaxoSmithKline, sertraline is marketed as Zoloft by Pfizer, and fluoxetine is the generic name for Prozac, made by Eli Lilly, which funded the work. (Prozac recently lost its patent protection, and a generic version is available for that drug but not for the other two.)

"There's often a belief that drugs in the same class are likely to be similar. But there are often few head-on-head studies" of drugs within the same class, says Dr. Kurt Kroenke, a professor of medicine at Indiana University School of Medicine in Indianapolis and lead author of the study, which appears in the Dec. 19 issue of the Journal of the American Medical Association.

Kroenke's research team evaluated the three groups over the next nine months, rating their symptoms of depression on a standard scale. They also assessed each individual's ability to function at work and with family and friends, and looked at such factors as sexual performance, pain, sleeping habits and memory.

By the ninth month, the adults in all three groups had gained between 15 and 17 points, on average, on the mood scale, a significant increase but an insignificant range, the researchers say. At the same time, the number of those who suffered serious side effects, like sexual dysfunction, or switched drugs or dropped off the medication was equal in all three groups.

Overall, about a fifth of the adults switched drugs at least once, and by the end of the study about half were still taking an antidepressant. More than two-thirds showed some improvement, and about 80 percent reported being satisfied with treatment, a percentage that didn't depend on what drug was used.

Most of the results weren't surprising, says Kroenke, who is also a scientist at Indianapolis' Regenstrief Institute for Healthcare.

But the researchers did show that the adults responded equally to the three drugs regardless of whether they had signs of anxiety before starting the pills, which is a factor generally thought to influence progress with certain SSRIs.

The findings are likely to encourage health insurance plans that urge doctors to prescribe the cheapest antidepressants.

"This study tells us. . .what we can expect if people take one of these three medications: On average, in the end you'll come out in the same place," says Dr. Gregory Simon, an investigator at the Center for Health Studies. The Seattle-based center is the research arm of Group Health Cooperative, a pre-paid health plan in Washington State.

But that doesn't mean the three drugs are identical, he adds.

Simon, who wrote an editorial accompanying the journal article, says that a small fraction of people -- 10 percent to 15 percent -- who use the compounds are taking other medications that can react with the antidepressants in curious and possibly harmful ways. And genetic variation is another important, and poorly understood, factor in how well patients will respond to SSRIs both as a group and individually, he adds.

Ultimately, Simon says, the study doesn't argue that everyone with depression should take generic Prozac.

"But the outcomes appear to be on average the same, so it is reasonable for someone to recommend the least expensive option. How much the drugs cost is a temporary phenomenon and may vary from place to place. It is probably true that generics will be cheaper," he says, "but it isn't necessarily true all of the time."

Both Simon and Kroenke have received funding from Eli Lilly.

What To Do

For more on depression, try the Depression and Related Affective Disorders Association.

For information about how generic drugs make treating depression and other conditions less expensive, visit About.com.