The men were taking part in a trial to test a new pain reliever at a lab in Rennes, France, along with 84 others.
There’s thought to be no known antidote to the drug.
The trial – which has now been suspended - involved healthy volunteers taking an oral pill.
Clinical trials are key to finding successful new treatments for diseases such as cancer, but they can have disastrous consequences.
A representative for the Medicine and Healthcare products Regulatory Agency, which regulates medicines in the UK, tells Closer: "Clinical trials in the UK have an excellent safety record and play a vital role in the development of medicines.
"Safety problems associated with clinical trials are rare.”
Closer's writers go head-to-head: Should we test medical drugs on animals?
But shocking figures reveal that 7,187 people in the UK have suffered unexpected adverse reactions to drug trials in the last five years.
Of these, 493 reactions were immediately life-threatening and 197 resulted in ‘significant disability or incapacity’.
Someone who knows how dangerous clinical trials can be is David Oakley, 44, who took part in the infamous 'Elephant Man' trial, which nearly cost him his life in 2006.
David was paid £2,000 to take part in the trials at London’s Northwick Park Hospital but, within hours of being injected with the drug TGN1412, his head swelled and his organs began shutting down.
Nav Modi, Mohamed Abdalla and Ryan Wilson, as well as two other men who weren’t named, also took part in the trial and were adversely affected, with Ryan losing the tops of his fingers and all of his toes, leaving him disabled.
Thankfully, they all survived but David still suffers physically and mentally, and is at high risk of developing cancer.
David, who lives with his wife Katrina, 39, and his three children, Hayley, seven, Henry, five, and Hannah, two, in Canada after emigrating in 2009, says: “Despite knowing the risks, I didn’t think anything would go wrong during my trial.
“I nearly died and my whole life has been affected. I’m still worried about what the future holds.
“The tragedy in France has brought back painful memories. I’m shocked drug trials are still going to horribly wrong.”
To read more of David's story, get your copy of this week's Closer magazine.