therapeutic index
The therapeutic index is a single number for 'how much room for error does this drug give me?' It compares the dose that causes harm with the dose that helps: a big number means a wide buffer, a small number means you are walking a tightrope.
Classically it is defined as LD50 divided by ED50 in animal studies, or more usefully in humans as TD50 divided by ED50. A therapeutic index of 100 means the toxic dose is a hundred times the effective dose — very forgiving. An index near 2 or 3 means a dose that barely doubles can become dangerous, so the drug is called narrow-therapeutic-index and usually requires therapeutic drug monitoring.
Its limitation is that it relies only on the two 50 percent midpoints and ignores the slope and overlap of the curves. Two drugs can share the same therapeutic index yet differ greatly in real-world safety if one has a steep curve crowding effect and toxicity together. For that reason stricter measures such as the margin of safety and the certain safety factor were introduced.
Penicillin has a very high therapeutic index, which is why doses can be generous; digoxin's index is so narrow that doses are measured in micrograms and serum levels are monitored.
Wide vs narrow: same idea, very different caution.
Narrow-therapeutic-index drugs — warfarin, digoxin, lithium, phenytoin, aminoglycosides — are the everyday reason clinicians order blood levels: a small dosing error can cross from cure to toxicity.