I have heard the claim that it is almost impossible to succeed as a professional bicycle racer without taking drugs because everyone else is taking it.
Is it true?
This is a very hard question to answer - how do you prove that a given cyclist isn't taking some undetectable drug?
What you can say is, in recent years, cycling has developed one of the most rigorous testing programs in all sports. Each professional cyclist has their own biological passport which tracks markers associated with drug use - the point is not to test for known drugs but for the likely effects of doping. If an athlete has a sudden rise in red blood cells, or makers associated with growth hormone, they're investigated or stood down.
A particular case in point is Brad Wiggins surprise result in the 2009 Tour de France. When he rode better than most people expected, he was able to show a long-term series of blood results which didn't show any evidence of doping
In the end, you will have to make your own judgement, but I think the level of testing used in modern professional cycling, and the fact only a relatively small number of cyclists are found guilty of doping, makes it reasonable to infer that most of the peleton are not cheating.
Willy Voet, a physiotherapist, claimed as much in his book Breaking the Chain: Drugs and Cycling: The True Story. The book contains a detailed account of a wide range of drug abuses, in the period leading up to the nineties. It is, btw, thoroughly recommended.
Circumstantial evidence against Lance Armstrong is outlined in this book : LA Confidentiel
And, of course, most recently Contador, Floyd Landis, Michael Rasmussen, Team Astana.
To say "most professional cyclists"? Well, I guess you'll never know. But the sheer number that have been caught is very high in both absolute and percentage terms. All may protest their innocence, but they were caught with banned drugs in their system - it's quite simple.
A comprehensive list is here, and a quick count shows that in 2006, for example, 42 separate cases occurred throughout the year. The biggest court case, as fair as I know, is Operación Puerto carried out in Spain