The test was part of a phase 1 clinical trial, the point at which a drug is given to humans for the first time. It is generally the riskiest. Prior tests are done in laboratories and on animals.

The healthy college student ended up in a London hospital fighting for his life.

He was one of six men to take the drug in a trial run by Waltham- based Parexel International. All ended up in the hospital, two of them in serious condition. Two others in the clinical trial took placebos and were unharmed.

Risks Can't Be Eliminated

Flanagan's family, according to The Mirror of London, were told he couldn't breathe, his head and neck had swollen to three times their normal size, and his legs had turned purple. An otherwise "healthy young man," Flanagan had seen the drug trial advertised on the Internet.

Such devastating side effects are extremely rare, but they highlight the dangers of testing drugs on humans, say researchers.

"I think tremendous progress has been made in minimizing the risks, but those risks can never be eliminated," said Dr. Steven C. Schachter, a Harvard Medical School professor of Neurology and a neurologist at Beth Israel Deaconess Medical School.

Parexel was managing the trial on behalf of German-based TeGenero AG, which developed the drug. The volunteers are being cared for at the Northwick Park Hospital while regulators try to figure out what went wrong.

One man's girlfriend said he looked "like the Elephant Man."

"He is like a shell of who he is and this machine is pumping out his lungs," Myfanwy Marshall told BBC News 24.

Supposed to Stimulate Immune System

The test was part of a phase 1 clinical trial, the point at which a drug is given to humans for the first time. It is generally the riskiest. Prior tests are done in laboratories and on animals.

TeGenero's drug showed no signs of safety problems in previous testing, said the company's chief scientific officer Thomas Hanke. It is supposed to stimulate the body's own immune system. In the volunteers, it caused an inflammatory response, shutting down their organs.

In the United States, all trials are reviewed by the US Food and Drug Administration and by the institution in which they take place.

Still, those considering volunteering should thoroughly review the consent forms and make sure they understand the potential risks, Schachter said.

"Patients need to understand that clinical research is an important part of developing therapies, but they're not proven," he said.