If you look at a chart of when EPO became available, and all those good knowledgeable DDR sport med docs became available on the free market, there's this discontinuous drop in world records fro pretty much all endurance sports.
Notice the discontinuity around 1994: https://en.wikipedia.org/wiki/10,000_metres_world_record_pro... https://en.wikipedia.org/wiki/5000_metres_world_record_progr... https://en.wikipedia.org/wiki/World_record_progression_1500_... https://en.wikipedia.org/wiki/Tour_de_France_records_and_sta...
You'll see similar changes with the introduction of synthetic testosterone in weight lifting records, and the physical appearance of the top bodybuilders.
EDIT: And this ignores "mechanical" doping in cycling- where you stick an electric motor on the bike. We've suspected it for a while at the pro ranks, and got confirmation when a small fry got nabbed: https://en.wikipedia.org/wiki/Femke_Van_den_Driessche