Read this:
And why does your veterinarian always want to sell you Hill's Science Diet?
Well, you should know that the associations, in general, have a vested financial interest in many of the products a veterinarian can sell to clients (Veterinary Associations are major shareholders in Hill's Science diet...), and they are paid by the manufacturer for promoting those products... (In fact, the manufacturers often have to bribe those associations to accept at least condoning their product, as the associations have the power to ban their members from using them! There are many examples of this happening, both in North America and Europe.)
The pet food manufacturers and the pharmaceutical companies also run "incentive programs" for practicing veterinarians. Those programs are basically giving the vet a nice commission of all sales, but in addition to a plain commission, the vet gets some significant cash bonuses when his/her sales reach certain numbers! This is of course, an excellent incentive for the vet to sell more of that product - whether or not the clients actually need it... (also read this page for more on pushing Hills; SD; Iams and Eukanuba)
Why Your Veterinarian Cannot Afford to Always Tell You the Truth