“I was deeply moved by their suffering,” Ms. Touraine said after visiting the patients and their families.

The drug was administered orally to healthy volunteers as part of a Phase 1 clinical trial by Biotrial, a drug evaluation company based in Rennes, on behalf of a Portuguese drug manufacturer, Bial. The drug is intended to help with mood, anxiety and motor problems linked to neurodegenerative diseases by having an effect on the endocannabinoid system, a set of brain receptors. Of 128 participants, 90 were given the drug, and the rest a placebo.

Experts in clinical trials said serious injuries involving early-stage clinical trials were rare but must be thoroughly investigated since they typically involve healthy subjects who would not otherwise have fallen ill.

Carl Elliott, a bioethicist at the University of Minnesota, said investigators should look into questions like how much the men were paid and whether they properly consented to the trial. “Many Phase 1 trial volunteers are poor and unemployed, and they volunteer for trials like this because they are desperate for money,” he said. “This means they are easily exploited.”

In a statement, Biotrial acknowledged “serious adverse effects” in a trial, adding: “The trial has been conducted in full compliance with the international regulations, and Biotrial’s procedures were followed at every stage throughout the trial, in particular the emergency procedures for the transfer of subjects to the hospital. We are in close and regular contact with the health authorities and ministry in France.”