Clinicians are urging people not to be deterred from volunteering for medical research after men in a drug trial fell seriously ill.
All medicines are tested on humans before being widely used
Two men are critically ill and four others are in a serious condition after being the first people to take an anti-inflammatory drug.
But researchers say human trials of new drugs are crucial in their development.
Experts have stressed that the severe reactions to this drug are unprecedented.
TeGenero, the company which has developed the drug, said the reactions in the men were completely unexpected, given earlier findings from animal and laboratory work.
Its chief scientific officer Thomas Hanke said the company's first concern now was making sure the patients got the best treatment possible and to support the families.
Professor Robert Moots, a rheumatologist at the University of Liverpool, runs clinical trials.
He said they were fundamentally important to get better drugs.
"That's not just to get more effective drugs, but also their safety."
"But I'm concerned that this horrible thing that's happened will put people off.
"I've never seen anything like this before."
'Safeguards are in place'
Professor Janet Darbyshire, who is director of the clinical trials unit of the Medical Research Council, told the BBC: "Clearly this is very worrying and we hope that the end result is going to be positive.
"But I think that the most important thing we must remember is that this is a very, very rare occurrence and that thousands of people enter drug trials every day.
"Without such trials, we're not going to have any new developments for treatment or vaccines," she added.
She said everything was done to protect volunteers from unnecessary risk.
"We have many, many safeguards in place and all these safeguards are put there to minimise the risk.
"What we can't predict is the unpredictable," she said.
She said it was not yet known exactly what had happened in the Northwick Park trial, but added: "Every precaution is taken to minimise the risk but we can't remove the risk of things we truly can't predict."
'Somebody must be first'
Ben Hayes, of the Association of British Pharmaceutical Industry (ABPI), added: "All of the medicines from which we benefit today would have been through these tests.
"Such tests are essential and, at some stage, somebody has got to be the first to take a new drug."
And Dr Evan Harris, a Liberal Democrat MP, who took part in early trials of an HIV vaccine trial six years ago, said it was vital that volunteers and patients continued to take part in such trials.
"Such trials are a critical step in the generation of new treatments for human diseases.