Before a drug is sold and released to the public, what measures must scientists take to ensure the safety of a drug and why are these steps taken?

Phase I- The drug must be tested on animals and tissue culturesPhase II - The drug is tested on healthy volunteers in small dosesPhases III- The drug is tested on sick patients in small dosesSmall doses are used intially to check that the drug is not toxicEfficacy of the drug is monitoredThe trial is double blind neither doctor nor patient knows who has the drugA placebo is given to some patients