In a crowded market like the one that antidepressant drugmakers are in, a journal article that analyzed more than 200 clinical trials and concluded that no antidepressant is better than any other wasn't exactly what the companies were hoping for.

Drugmakers rightly try to differentiate their products, but a just-published article in the Annals of Internal Medicine concludes that there's no reason to choose one second-generation antidepressant over another on the basis of differences in effectiveness.

Not what the doctor ordered. But it's a big deal, because there's a lot of money in this sector.

Drug

Manufacturer

YTD Sales (in Millions)

Zoloft

Pfizer (NYSE:PFE)

$408

Cymbalta

Eli Lilly (NYSE:LLY)

$1,976

Paxil

GlaxoSmithKline (NYSE:GSK)

$702

Wellbutrin

GlaxoSmithKline and Biovail (NYSE:BVF)

$538

Lexapro

Forest Labs (NYSE:FRX)

$1,744

Unfortunately for these branded drugs, the study also included antidepressants available as generics, such as Eli Lilly's Prozac, Wyeth's (NYSE:WYE) Effexor, and Bristol-Myers Squibb's (NYSE:BMY) Serzone. Since they're usually available more cheaply, doctors may continue to gravitate toward prescribing generics.

Of course, effectiveness of the drugs is just one component to choosing a drug -- side effects and, to a lesser extent, ease of use also come into play. The article concluded that the side-effect profiles were relatively similar, but the drugs do have different dosing regimens -- some are available in extended-release formulas -- which could be a selling point to patients.

And it should be noted that some antidepressants are used to treat other diseases. Cymbalta, for instance, is also prescribed for diabetic nerve pain and fibromyalgia. This study did not address that issue, so sales for those reasons should be unaffected.

When a product is clearly different or better than its competitors, a company doesn't have to do as much to sell it. But when it's just the same? Hello, advertising (and the related expenses).

Overall, though, this article has to be a major downer for drugmakers and investors. At least they won't have to waste too much time figuring out which antidepressant to take.

Pfizer, Lilly, and Glaxo are all Income Investor recommendations. To see how dividend-paying stocks can offer both secure income and the opportunity for growth, take a free look at this newsletter service with a 30-day free trial.

Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. Pfizer is also an Inside Value recommendation, and the Fool owns shares, too. The Motley Fool has a disclosure policy.