Six men who became ill during a drug trial remained in serious condition in a London hospital Thursday.

Two of the men were listed in critical condition, Northwick Park Hospital said in a statement.

Raste Khan — one of two men given a placebo in the trial — said the six had been stricken within a few minutes of receiving the drug.

"Everyone was continuously vomiting," Khan said in an interview broadcast Thursday on Sky News.

They were "fainting, coming back to consciousness. Again I assume they had headaches, because a lot of them were kind of like holding their heads," he said.

One man was screaming and complained of pain in his back, Khan said.

The six men had been given does of TGN1412, a monoclonal antibody developed by TeGenero AG of Wuerzburg, Germany, for treatment of autoimmune and inflammatory diseases and leukemia.

"We are continuing to treat the inflammatory disorder triggered by the drug and the resulting effects," said Dr. Ganesh Suntharalingham, clinical director of intensive care at the hospital

Dr. Thomas Hanke, TeGenero's chief scientific officer, said Wednesday that TGN1412 had not caused any problems in previous testing.

Khan said the reaction set in quickly.

"Roughly maybe a few minutes after the final person had been given the drug, the first person who had been given started to kind of shake, he took his top off, looked like he was kind of burning up," Khan told Sky.

"Obviously, I assume he had a headache because he was rubbing his head."

Khan said most of the patients were "moaning and groaning," and only one said what hurt.

"He was saying, 'My head hurts.' He was going, 'Doctor, my head!' 'My back!' 'Doctor, I need help! I want help! I can't breathe!'

"He was just literally shouting randomly all the things he must have been going through, so many different types of pain," Khan said.

"I feel guilty because I was the lucky one," Khan added, "whereas other people obviously were not so lucky."