San Jose, Calif. ? In a place like Silicon Valley, where career prospects often hinge on a person’s intelligence and ability to work hellishly long hours, “brain doping” probably was inevitable.

The expression refers to a growing national trend that troubles some medical ethicists, in which pills designed to treat sleep and mood disorders are being popped by healthy people to help keep them mentally sharp and boost their productivity.

In April, the prestigious journal Nature reported that one-fourth of the 1,400 people responding to an informal survey admitted they had taken drugs for nonmedical reasons to improve their concentration or memory. The medicines included Ritalin, a stimulant for treating attention-deficit hyperactivity disorder; Provigil, for sleep disorders; and so-called beta blockers for cardiac arrhythmia.

The practice appears to have gotten at least a foothold in Silicon Valley, especially with Provigil, a prescription medicine approved for narcolepsy, obstructive sleep apnea and shift-work sleep disorder.

Doctors can legally prescribe a medicine for off-label uses. But Casey McEnry, spokeswoman for the U.S. Drug Enforcement Administration’s San Francisco office, said it is illegal to sell Provigil, give it away, buy or use it, without a prescription.

Using the drug without a prescription also can be hazardous. Cephalon, Provigil’s Pennsylvania manufacturer, notes that Provigil “may cause you to have a serious rash or a serious allergic reaction that may result in hospitalization or be life-threatening.”