But the act itself is so rare  1 in 10,000  that a series of drug trials cannot pick up enough cases to allow for adequate analysis. A drug trial typically lasts weeks to months and may include, at the high end, little more than a couple of hundred patients. In the case of the epilepsy drugs, the F.D.A. found 4 suicides among some 44,000 people taking the drug in 199 studies, and none among some 28,000 on placebo. Doctors would have to treat about 500 patients before seeing one case of suicidal thinking or behavior that would not have occurred without the drug.

The agency is now requiring that manufacturers in their studies track suicidal symptoms. But drug makers traditionally have had little incentive to do so; on the contrary, in many studies scientists try to screen out suicidal patients and bury any mention of suicide attempts deep in their reports, or with vague language.

To make up for the tiny number of completed suicides, health regulators have used suicide signs, or markers. But these are not well understood, either. One of them is suicidal thinking, or “suicidal ideation.” This is recorded in a study when a patient tells a doctor that he or she is feeling suicidal.

It hardly takes a psychiatrist to point out that the act can’t happen without the idea. But having the idea very rarely leads to the act, as psychiatrists, psychologists and almost anyone who has been a teenager can attest.

Is the person who tells the doctor about the dark thoughts somehow more at risk?

No one knows. “Every psychiatrist with a big practice will have a few suicides, and you’re going to have people who don’t say anything about it  and are very much at risk,” Dr. John Davis, a professor of psychiatry at the University of Illinois at Chicago, said.

Lanny Berman, executive director of the American Association of Suicidology, said in an interview that research suggests that about a quarter of suicides are impulsive: the idea strikes and the person acts quickly. Studies of hospitalized patients have found that many who go on to take their own lives deny to doctors any thoughts of it, he said. “We just don’t know enough about the relationship” between the thoughts and the behavior, Dr. Berman said.

Image UNKNOWABLE The poet A. Alvarez called suicide a closed world with its own irresistible logic. Credit... Jonathan Player for The New York Times

Not to mention that people who are thinking about it more often talk themselves out of the act, also on a sudden whim. As the G. K. Chesterton poem “A Ballad of Suicide” has it,