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Phase I clinical trials exist for this purpose. The objective of Phase I trials is to establish safety, dosage, and side-effects of drugs in human subjects, and to observe their proposed mechanism of action if relevant.
It's rare for Phase II and III trials (which have clinically-relevant endpoints ) to be carried out on healthy subjects. Part of this is ethical considerations, but also clinical trials are extremely expensive to carry out, and there's not much payoff in learning whether your drug has some specific effect on healthy subjects.