Five people have become seriously ill — with one described as brain dead — after taking part in a clinical trial for an experimental drug earlier this week. The aftermath from the experiment, which took place at a private clinic in France, brings to light how dangerous drug trials really are.

The phase I clinical trial was reportedly to test the safety of a cannabinoid-based painkiller, according to The Independent. These claims, however, have since been denied. What is known, is that the trial was conducted by Biotrial, a research company specializing in carrying out drug trials for clients. The Portuguese pharmaceutical company Bial reportedly developed the drug.

A total of 90 people, aged 30 to 50, were given the drug in various doses, while others were given a placebo. All participants were healthy prior to the study, which took place in a private research center. After the participants began experiencing adverse effects to the oral medication, the trial was suspended and all participants underwent medical treatments, including those who took placebos.

Clinical trials are necessary in order to bring new drugs to the market. But before any new drug can be given to patients, it must first undergo extensive testing for both safety and effectiveness. These trials often last at least a year and involve hundreds, if not thousands, of volunteers. Though incidences like this are rare, they’re not unheard of.

Ten years ago, a clinical trial for an experimental immune stimulant nearly killed all six volunteers. Within minutes of taking the experimental drug, volunteers were overwhelmed with chills, pain, and nausea, The New York Times reported.

“It started off with a lower back pain,” Ryan Wilson, one of the men involved in the trial told The Sunday Times. “I just remember it getting worse and worse, and then I remember vomiting, and then just too much pain and I just blacked out. And that was it. Gone.” Wilson experienced the most adverse effects from the medication; he was in pain for months, and even lost parts of his fingers and toes to the effects of the drug.

Biotrial claims it conducted the trial in full compliance with international regulations. “Biotrial's procedures were followed at every stage throughout the trial, in particular the emergency procedures for the transfer of subjects to the hospital,” the company said in a statement, according to Sky News. The French Health Authorities and Ministry has begun a full investigation in order to understand what went wrong.