Speaking from hearsay and not experience, my 'guess' is that drugs, being illegal need code names. Some drugs had male names; e.g. Billy, BillyWhiz (cartoon character) as in speed = amphetamine due to it's stimulating effect. 'Harry (the Horse) was heroin but due to 'aitch dropping', Harry became 'Arry and got confusing in case they meant 'Arry Acid' which they didn't, cos that's Sid so they just called it 'H'. Cocaine became simply 'Charlie' because it sounded better than 'Colin'.
So that's Billy, Sid, Harry and Charlie. Other drugs don't have boy's names (unless you know differently.).