<prompt> Comparison of efficacy of a new drug B with an existing drug A is done in which phase of clinical trials?Phase IPhase IIPhase IIIPhase IV  <response>: The purpose of phase III trials is to obtain adequate data about the efficacy and safety of drugs in a larger number of patients of either sex in multiple centres usually in comparison with the standard drug. Phase I: Human Pharmacology Non-therapeutic trials: Done in healthy persons Phase II: Therapeutic exploratory trials: conducted in patients to determine therapeutic effects, effective dose range and fuher evaluation of safety and pharmacokinetics in patients Phase III: Therapeutic confirmatory trials: Conducted to obtain adequate data about the efficacy and safety of drugs in a larger number of patients of either sex in multiple centres usually in comparison with the standard drug. Phase IV: Post marketing studies 