Although I would agree that vets overcharge for drugs quite disgracefully,I am very much against many drugs being bought over the counter. Penicillin can, partucularly in young animals, set up an irreversible scouring resulting in death, and Bute - already widely misused - can affect the fertility of mares, in some cases I believe resulting in sterility.
If steroids were freely available there would be considerable abuse and the same applies to growth hormones. These drugs can artificially improve the appearance of an animal but can also play merry hell with its reproductive system.
I remember talking to a cattle dealer some years ago when steroids were widely used, who told me that of the ten bulls he bought at Perth bull sales, eight of them tested positive to steroids. I have also seen two exceeding moderate hunters transformed by the use of steroids into magnificent animals - these belonged to a vet.
Cortisone is another dangerous weapon - it can mask all manner of lameness in horses and used to be very popular among unscrupulous vendors at horse sales. The unsuspecting buyer was of course badly stung when his purchase turned out to have incurable and progressive navicular disease
Unfortunately there are too many people around who will definitely use drugs for the wrong reasons and many others who will use them without sufficient knowledge but with the best intentions - resulting in disaster.