The business of depression is particularly lucrative.

Whereas other costly conditions, such as heart conditions or cancer, tend to strike late in life, most people experience depression when they’re much younger, usually between the ages of 15 and 30. Besides setting in early, depressive episodes tend to recur.

Drug companies realize the importance of resonating with customers early and often, in hopes of establishing loyalty from those who need to take the pills over a long period — so much so that they’ve been willing to give the pills away. In May, Pfizer announced that it would offer many of its brand-name drugs — including the popular antidepressant Zoloft — free to people who had lost their jobs and health insurance. While the campaign was marketed as sympathetic, another aim was probably less charitable: to keep those patients from switching from Pfizer brands to cheaper generics.

Rising reliance on antidepressants doesn’t benefit Big Pharma exclusively, though. Drug companies are notorious for big marketing campaigns — bolstering revenue streams for businesses that rely on those ads.

“Certainly they’re contributing big time to advertising revenue,” says Charles Barber, author of “Comfortably Numb: How Psychiatry Is Medicating a Nation.”

In the first quarter of 2009, as automotive ads — long the top advertising category in the Unites States — plummeted by 28 percent, according to Nielsen rankings, pharmaceutical companies’ ad spending was more consistent. It still dropped, but only by 11 percent. Drugmakers were the third-biggest spender of ad money in that period. Without those purchases, some media outlets already floundering in the thinned-out ad market would have been much worse off. Because most depression sufferers are women, female-targeted lifestyle magazines get a particular boost from companies pushing antidepressants.

The most important benefit that antidepressants can provide, of course, is to those taking the medications. While the salutary effects are just the relief that some people need, a few skeptics have theorized that the pills may change a person’s mind-set too much.

Nearly a decade ago, Randolph Nesse, a professor of psychiatry at the University of Michigan, suggested that investors numbed by antidepressants would take risky bets and make bad decisions.

They may “become far less cautious than they were before, worrying too little about real dangers,” he wrote. He predicted that, as more people turned to prescription medications, the collective effect would cause a Wall Street bubble to grow and burst.