Scientists, too. Anyone, really; fail faster, people! Celebrate the process of trial and error.
Engineers and doctors and nurses... Eh... Maybe not so fast...
First, establish that a drug binds to the target in laboratory conditions (relatively quick). After that, test the drug in an animal model. After that, proceed to clinical trials, which are phased (phase I, II, III) to first establish whether a dosage exists that is safe (Phase I), what an effective dosage is (Phase II), and only after than testing if it improves upon the current standard of care (Phase III).
Each phase is more costly (both in time and money) than the previous phase and will eliminate a significant portion of drug candidates.
"The success rate of each drug discovery stage in academia was 31.8% for preclinical, 75.1% for phase I, 50.0% for phase II, 58.6% for phase III."