The gap between the dose that helps and the dose that harms — a wide gap means more room for error; a narrow gap means precision dosing is critical.Formally: the ratio between the toxic dose (TD50) and the effective dose (ED50). Acetaminophen has a relatively narrow therapeutic index; penicillin has a wide one. For most research peptides, the therapeutic index in humans is genuinely unknown because systematic dose-ranging and toxicology studies in humans do not exist. Community dosing protocols that use animal-derived doses assume a therapeutic index that has not been characterized in human subjects.