Because reactions to hallucinogens can vary so much depending on the setting, experimenters and review boards have developed guidelines to set up a comfortable environment with expert monitors in the room to deal with adverse reactions. They have established standard protocols so that the drugs’ effects can be gauged more accurately, and they have also directly observed the drugs’ effects by scanning the brains of people under the influence of hallucinogens.

Scientists are especially intrigued by the similarities between hallucinogenic experiences and the life-changing revelations reported throughout history by religious mystics and those who meditate. These similarities have been identified in neural imaging studies conducted by Swiss researchers and in experiments led by Roland Griffiths, a professor of behavioral biology at Johns Hopkins.

In one of Dr. Griffiths’s first studies, involving 36 people with no serious physical or emotional problems, he and colleagues found that psilocybin could induce what the experimental subjects described as a profound spiritual experience with lasting positive effects for most of them. None had had any previous experience with hallucinogens, and none were even sure what drug was being administered.

To make the experiment double-blind, neither the subjects nor the two experts monitoring them knew whether the subjects were receiving a placebo, psilocybin or another drug like Ritalin, nicotine, caffeine or an amphetamine. Although veterans of the ’60s psychedelic culture may have a hard time believing it, Dr. Griffiths said that even the monitors sometimes could not tell from the reactions whether the person had taken psilocybin or Ritalin.

The monitors sometimes had to console people through periods of anxiety, Dr. Griffiths said, but these were generally short-lived, and none of the people reported any serious negative effects. In a survey conducted two months later, the people who received psilocybin reported significantly more improvements in their general feelings and behavior than did the members of the control group.

The findings were repeated in another follow-up survey, taken 14 months after the experiment. At that point most of the psilocybin subjects once again expressed more satisfaction with their lives and rated the experience as one of the five most meaningful events of their lives.