As far as I know, drugs have always played a part in high level sport, with cycling being no exception. The difference between drugs of today and drugs of decades past is also worth noting. The old timers had what could be called dynamic drugs or drugs which excite or stimulate the body. Large doses of caffeine, amphetamines or uppers existed and were frequently used by all kinds of athletes. Painkillers have also been available for many years and for many sports, perhaps cycling included. They could have conceivably aided an athlete like Harris and others. Some painkillers could assist but could also hinder high level performance. Cocaine has so many "distillation breakdown products" that are used to this day to kill pain, with minimal adverse effects on overall power and endurance outputs.
Here's the difference between days of yore and drugs of the last coupl' a decades.... Simpson stimulated himself right into the grave. His stimulants no doubt assisted him at times and he no doubt competed with other caffeinated and amphetamined up cyclists, but he was unfortunate to have "overdone it a bit". Painkillers kill the pain of over exertion and stimulants improve output but nothing compares to what we have at our disposal now. EPO is a direct zap right into the body's ability to deliver oxygen to the cells (As one pro told me ... EPO gives you three gears without any extra sweat). Steroids help the body improve muscle mass and can be used in small or large quantities as needed to improve power without undue weight increase. HGH or growth hormone improves the body's ability to recover and repair and can improve the integrity of the core organ and ligimental-bone structure. Testosterone patches can be applied to the scrotum for a quick overnight aid in recovery. IV's (intravenous) help the body rehydrate and replenish mineral and fuel supplies without taxing the digestive system. This is especially useful in multiple stage races like the TDF or Giro etc.
Nothing the old timers had whether legal or illegal could assist them the way the "moderns" are pumped up, "freshened up" and fluffed. The desire to cheat is the same now as it always was, but the simple truth in my opinion is that even if the old timers wanted to, their ability to cheat was severely limited compared to nowadays. This is why, whether the sport be cycling or baseball or weightlifting, or gasp "bodybuilding", the modern athletes should have a real or mental notation of an asterisk next to just about anything they do. For me, the records of the old timers stand not because they wouldn't have cheated because they were such fine fellows, but because they couldn't have cheated the same way the cyborg athletes of today can and may I be so bold as to say .. do.
Garth Libre in Miami Fl, recovering nicely from a serious staph infection of one finger.