The therapeutic index (TI) is a ratio between the LD50 (lethal dose at which 50% of test subjects die) and the EC50 (effective concentration 50%, the concentration of drug giving half the maximal biological response). TI is used to determine the safe dose that should be administered, without having too large of a risk of death.
It is important to determine the safety of a drug through in vitro and in vivo drug toxicity studies. These allow risk to be reduced at each stage of the drug trials process, while determining the risk the compound of interest has.
Thalidomide, a drug previously used for morning sickness, underwent clinical trials to test its safety. As no immediate issues were found, it was approved for clinical use. However, due to the short duration of trials, adverse reactions that took time to develop did not become apparent until it was being prescribed. The drug was withdrawn from the market after birth defects were found to be more common where the drug was used. This was due to two enantiomeric forms of the molecule being present, and the interconversion between the two in the body.