If 10mg of drug "x" exerts the desired pharmacological effect, but 160 mg of "x" is a fatal dose, what is the therapeutic index of "x"?

The therapeutic index of a drug is also known as the therapeutic window or safety window, and compares the therapeutic effect of a drug with the amount related to toxicity or adverse side effects.

This is calculated as TD50:ED50. TD50 refers to a dose of the drug that causes toxicity or adverse side effects in 50% of the subjects. ED50 refers to a dose of the drug that leads to the desired pharmacological outcome in 50% of the subjects. This means that drugs with higher therapeutic indices are preferable because it requires a low dosage for efficacious results and/or a high dose for side effects to occur. This is referred to as a large therapeutic window because there is a large dosing margin where the drug can be administered without causing undesired side effects.

For drug "x" in particular, the therapeutic index will be 160mg:10mg, which is equal to a 16:1 ratio.