The paper consists of three parts. Part I lists the assumptions about alcohol made by judges and legal academics and traces those assumptions back to their medical and ideological roots in the 19th century. Part II then contrasts those assumptions with the scientific evidence on the relationship between psychoactive drug use and violent behavior, recklessness, intention, cognitive ability, memory, automatism, and insanity. The evidence presented in Part II indicates that important discrepancies exist between the empirical evidence and the legal assumptions about intoxication. Relevant cases are canvassed to demonstrate that the facts in such cases are consistent with the scientific findings but often inconsistent with the legal decisions. Finally, Part III analyses the function in criminal court served by medical testimony on intoxication. It is suggested that medical testimony on this issue is frequently biased and inappropriate and that such testimony should be minimized or eliminated entirely.