"Long-Acting" Penicillin in Horses
"Long-acting" penicillin, a combination of benzathine penicillin and procaine penicillin, is available for use in horses. The idea is that the drug releases penicillin slowly into the body so that a single injection lasts a few days. Good in theory - but not in reality.
The problem with long-acting penicillin is the penicillin levels achieved in the body (i.e. the concentration of penicillin in the blood and other tissues) aren't actually high enough to do anything (except perhaps to help encourage the development of penicillin resistance among bacteria). There are some exquisitely susceptible bacteria that might be killed by long-acting penicillin, but the vast majority of horses that get better after treatment with long acting penicillin probably get better despite treatment (i.e. they would get better on their own anyway).
If you need to use an antibiotic, use an effective one that has been prescribed by your veterinarian. Don't use long-acting penicillin.
As a paper looking at the pharmacokinetics of penicillin in the horse (Love et al, 1983) stated, "Fortified benzathine penicillin appears to have little value for antimicrobial therapy in the horse." That says it all.
