Praise be to Allah.
We put this question to Shaykh ‘Abd-Allaah ibn Jibreen (may Allaah preserve him, who answered as follows:
If the harm which may result from this trial of the new medicine can be treated with permissible (halaaal) medicine and its effects can be lessened, whether this treatment is administered by the drug company or by others, then it is OK for him to use the new medicine in order to test it, whether this is done in return for money or voluntarily. But if it is known that the medicine may have irreversible harmful side effects which cannot be treated, then it is not permissible for him to take it, even if a lot of money is offered in return, because Allaah says (interpretation of the meaning): “And do not kill yourselves” [al-Nisaa’ 4:29]. And Allaah knows best.