Three is Not Better Than one
With the increased professionalism of veterinary practice there is much less of the old backyard manufacturing of pharmaceuticals, generally resulting in higher quality products that are under the auspices of regulatory authorities. Back in the day it was common practice to include two or three different sulphonamides in veterinary powders, not to increase efficacy but reduce overall toxicity. The advent of trimethoprim in the mid-20th century obviated the need for that, resulting in highly efficacious and safe therapy. An historic case...