It used to be that penicillin or “pen-strep,” a combination of penicillin and streptomycin, was the antibiotic used for virtually every infection in a horse. Today, however, we have newer — and more expensive — antibiotic choices. As with most things, newer isn’t necessarily better, and you should think twice before automatically reaching for any antibiotic. Not only is the possibility of antibiotic resistance buildup a real concern, but you may find using the wrong drug worsens the illness. As always, consult with your veterinarian before using any type of drug.
Most of the antibiotics being used in horses have not been FDA-approved for use in horses (see chart), which means that formal safety and efficacy studies have not been done. While most have at least some formal investigation or widespread field use information behind them, there is rarely any clear indication to jump on the bandwagon of the trendiest antibiotic in favor those known to be safe and effective.
Bypassing the FDA-approval process is within the legal prerogative of a veterinarian treating an individual case, but caution is advisable without appropriate culture results to prove that only an unapproved drug will be effective. When using an unapproved antibiotic in your horse, you need to understand you are taking a risk regarding potential side effects.