The therapeutic index of a drug is defined as its toxic dose ($TD_{50}$) divided by its effective dose ($ED_{50}$). Why is it defined as a ratio rather than the difference $TD_{50} - ED_{50}$?
For example, if drug A has an effective dose of 1 ml and a toxic dose of 2 ml, and drug B has an effective dose of 50 ml and a toxic dose of 100 ml, then they both have the same therapeutic index (2), but intuitively, I would say drug A is much more dangerous, which is reflected if we take the difference instead.