Like most Peace Corps volunteers, Martin Giannini embarked on his mission full of high hopes and enthusiasm. His assignment in Togo promised to be the adventure of a lifetime. It certainly was-but not the kind he expected. Giannini's African adventure ended in a padded room in a Chicago psych ward. 'I was totally loony,' admits Giannini. 'It felt like I was in some 'X-Files' episode with instructions being planted in my brain. I tried to escape, but couldn't get past the four guards.' What led Giannini, a healthy young man with no history of mental illness, to take on a battalion of guards in a psychiatric hospital? A drug, say his doctors. An antimalaria drug the Peace Corps recommended.

Mefloquine, known commonly under the brand name Lariam, is the most prescribed malaria prophylaxis in the world. It's clearly the most effective. And controversial.

Like Giannini, an increasing number of Lariam users have reported hallucinations, paranoia, depression, nightmares and other psychotic effects after taking the drug. It has been implicated in suicide attempts and numerous aborted trips. In the last two years alone, the alleged side effects have led to British and U.S. lawsuits against Lariam's manufacturer (unresolved), a storm of media coverage (ongoing) and a Canadian government investigation into the military's use of Lariam in Somalia (pending). Tap into the global travelers' wire, and the word is clear: Take this drug at your peril.

Just ten years ago, Lariam was greeted by doctors as a chemical miracle. Strains of malaria in Africa and Asia had developed resistance to chloroquine-the drug of choice since World War II. The result was skyrocketing rates of infection. By the late 1980s, the Peace Corps considered abandoning its African operations altogether because half its volunteers were contracting malaria. Then came Lariam. Infection rates dropped overnight. Experts pronounced the drug a godsend.

How did Lariam go from wonder drug to dreaded drug in a few years? Is it an unsafe medication unleashed without adequate testing, as some argue-or is it the victim of a rumor mill run amok, as others contend?

There's little doubt that Lariam may cause side effects. The manufacturer, Hoffman-La Roche, warns against a litany of possible reactions, from hypertension to hallucinations. What no one can agree on is the risk. For severe psychotic reactions like Giannini experienced, previous studies indicate the rate is an acceptable one in 10,000. But a recent British survey pinned the figure at an alarming one in 140. That and a growing pile of travelers-gone-loco stories have convinced many people to question Lariam's safety.

A leader of malaria surveillance for the Centers for Disease Control and Prevention, Dr. Hans Lobel says simply, 'Mefloquine is a remarkable drug. It's 95 percent effective, and study after study shows it to be safe. Would you rather get malaria?'