Penicillin is a commonly available antibiotic that is frequently given to horses. There are several varieties of products and combination of products that all have their own dosages and characteristics that are important to understand for the proper treatment of infections. In general, penicillin is an antibiotic for treating gram positive infections. Penicillin is particularly effective against streptococcus infections like Strangles and uncomplicated wound infections. Many serious infections of foals and horses are often gram negative or mixed infections and require other antibiotics. This article is about the use, contraindications, toxic effects, dosages, and association drug rules that apply to this medication.

To know what type penicillin you have and what its concentration is you need to look at the ingredients on the bottle. Penicillin g potassium and sodium salts are rapidly absorbed after im injections and yield high peak levels usually within 20 minutes of administration. In horses, equivalent doses given either iv or im demonstrated that im dosing will provide serum levels above 0.5 Micrograms/ml for about twice as long as iv administration [approx. 3 - 4 Hours (iv) vs. 6 - 7 Hours (im)]. The procaine form is considered to be the best for IM use in the horse and it is usually available in a 300,000 IU / cc concentration. Procaine penicillin g is slowly hydrolyzed to penicillin g after im injection. Peak levels are much lower than with parenterally administered aqueous penicillin g sodium or potassium, but serum levels are more prolonged. Serum levels can be very prolonged, but levels attained generally only exceed MIC's for the most susceptible streptococci, and the use of benzathine penicillin g should be limited to these infections when other penicillin therapy is impractical.

There are some forms, frequently referred to as "long acting" preparations that contain the benzathine form of penicillin. Research has shown that it has too slow an absorption time to reach recommended therapeutic levels. The combination products with penicillin / streptomycin generally have too low a concentration of penicillin in streptomycin given too infrequently to do much good.

Combination products containing corticosteroids should not be used.