American doctors wrote more than 164 million prescriptions for antidepressants in 2008, making it the third-most-prescribed of any class of drugs. Now Bristol-Myers Squibb is tapping into that vast market with an aggressive advertising campaign for its blockbuster antipsychotic medication, Abilify.

Abilify was originally approved for treating schizophrenia and, soon after, bipolar disorder. But it hit the real jackpot in late 2007 when it won approval as an add-on treatment for people with major depression who haven't gotten adequate relief from taking an antidepressant alone. It's the first antipsychotic medicine approved by the Food and Drug Administration for that use.

Now the manufacturer of Abilify is advertising the drug's use in treating depression in print and on television, and it's working: U.S. sales of Abilify topped an incredible $2.37 billion last year, an increase of 33 percent from 2007, making it the 12th-highest earner among branded drugs.

While the ads may have worked to promote drug sales, the trouble is that anyone watching the TV ad would swear the drug is just another antidepressant. But Abilify isn't a common drug. It's a member of a class of drugs known as atypical (or "newer") antipsychotics. These medicines have different -- and, in some cases, considerably more serious -- side effects than the most common class of drugs used to treat depression, selective serotonin reuptake inhibitors, or SSRIs, which have plenty of safety concerns of their own.

Abilify's growing popularity hinges partly on the fact that it may be less likely to cause certain side effects, such as weight gain and elevated blood-sugar levels, than other drugs in its class, such as olanzapine (Zyprexa). But it still carries some risk of those side effects, as well as of restlessness, decreased white-blood-cell count and tardive dyskinesia, a condition marked by involuntary repetitive movements of the limbs and body that can become permanent.

In two studies sponsored by the manufacturer, 25 percent of patients who added Abilify to their antidepressant drug experienced akathisia (inner restlessness and urge to move around), compared with 4 percent of patients taking an antidepressant plus a placebo. And Abilify costs around 45 times as much per month as many antidepressants.

Consumer Reports reasons that Bristol-Myers Squibb is promoting Abilify for depression because only a limited number of people have schizophrenia or bipolar disorder. Depression, on the other hand, is a veritable cash cow among mental-health conditions. One in seven people will experience a depressive episode at some point in their lives. If you want to sell upwards of $2 billion a year of a drug, that's the kind of market you need.

The use of antipsychotics along with antidepressants in hard-to-treat cases of depression isn't new. But because of their more-severe side effects, antipsychotics are usually considered an option of last resort for depression, to be tried only after exhausting other options such as a different antidepressant, a different dose of the same antidepressant or a combination of two antidepressants together.

Not surprisingly, the Abilify advertising doesn't get into that. Quite the contrary, it seems to suggest that if the first antidepressant you try isn't enough, you can, and should, jump right onto Abilify as an add-on drug.

If this business of marketing an extremely powerful psychiatric drug on TV and in popular magazines such as People seems insidious to you, Consumer Reports shares your concerns. But there could be more to come. Perhaps inspired by Abilify's unbelievable success, the competing (and seriously side-effect-plagued) drug quetiapine (Seroquel) is now seeking approval as a depression add-on. What a very depressing battle of the ads that could be.