Rafael Palmeiro seems an unlikely Viagra pitchman. The Texas Rangers slugger is only 37 and won't admit to having erection problems, yet he recently agreed to appear in ads promoting the drug. The deal has made people wonder whether Palmeiro really represents men with erectile dysfunction, or whether Pfizer, the company that makes Viagra, wants to persuade young men to try it for fun.

It's true that erectile dysfunction is more common in older men, but many potential Viagra users are hardly senior citizens: About 40% of 40-year-old men in the U.S. have some degree of erectile dysfunction. Most Viagra users today, according to Pfizer, are in their early to mid 50s. So it makes sense that the company would want to reach more men around Palmeiro's age.

Urologist Myron Murdock, medical director of the Impotence Institute of America, says these men are likely to use Viagra because sexual performance is a high priority for them.

A younger man, Murdock says, "wants his V-12 Jaguar working just perfectly," whereas an elderly gent may be content with less dependable erections. What's more, the sexual partners of younger men "are more demanding of their performance," Murdock says.

Pfizer denies that it's promoting Viagra for recreational use. "We've consistently opposed that," says spokesman Geoff Cook. Nevertheless, Murdock says it's fine to pop the little blue pill to "optimize" your sexual performance.

We assume all young men have normal sexual functioning, "but they're really not normal," Murdock says. Hardening of the arteries, which restricts blood flow to the penis, can begin during the teen years, so that by the time a man is in his 20s, his ability to get and keep an erection has already begun to decline. Murdock says many men who seek Viagra for recreational use actually have minor erectile dysfunction.

There's also some evidence that Viagra can shorten the time it takes a man to recover after sex and be ready for another round. This is called the "refractory period." Normally it lasts 20 minutes or longer. One study, published in the journal Human Reproduction in January 2000, found that Viagra shortened the refractory period by about 10 minutes in healthy men.