Antibiotic resistance could be impeded by choosing narrow-spectrum antibiotic and correct dosage. Penicillin has narrow spectrum, is efficient, and the most valuable antibiotic used in horses. Infectious arthritis is devastating to the horse's function, risking irreversible damage unless treated promptly and effectively, but knowledge of the penicillin pharmacokinetics in the synovia is lacking. One aim of the project is to optimize intravenous natriumbensylpenicillin (pc) dosage for treatment of joint infections. Additionally, we aim to explore the feasibility of administering Na-pc combined with lidocaine intramuscularly as an alternative to penicillin procaine, providing a safe, painless treatment without procaine toxicity, thus reducing the use of alternative broad-spectrum antibiotics. Optimal dosage of pc leads to, aside from good treatment results, less influence on the normal bacterial flora, shorter treatment times and consequently less resistance.