Penicillin G is the drug of choice for infection caused by organisms susceptible to it, unless the patient is allergic to this antibiotic. However, use has declined very much due to fear of causing anaphylaxis. 1 . Streptococcal infections Like pharyngitis otitis media, scarlet fever, rheumatic fever respond to ordinary doses of PnG given for 7-10 days. For subacute bacterial endocarditis (SABE) caused by Strep. viridans or faecalis high doses (10-20 Ml. i.v. daily) along with gentamicin given for 2-6 weeks is needed. 2. Pneumococcal infections PnG is not used now for empirical therapy of pneumococcal (lobar) pneumonia and meningitis because many strains have become highly penicillin resistant However, PnG 3-6 MU i.v. every 6 hours is the drug of choice if organism is sensitive. 3. Meningococcal infections are still mostly responsive; meningitis and other infections may be treated with intravenous injection of high dose 4. Gonorrhoea :PnG has become unreliable for treatment of gonorrhoea due to spread of resistant strains. The treatment of ophthalmia neonatorum 5. Syphilis T. pallidum has not shown any resistance and PnG is the drug of choice. Early and latent syphilis is treated either with daily injection of 1 .2 M U of procaine penicillin for 10 days or with 1-3 weekly doses of 2.4 MU benzthine penicillin. For late syphilis, benzathine penicillin 2.4 MU weekly for 4 weeks is recommended. Cardiovascular and neurosyphilis requires 5 MU i.m. 6 hourly o f sod. PnG , . 2 weeks followed by the above regimen. Leptospirosis: PnG 1 .5 MU injected i. v. 6 hourly for 7 days is curative. 6. Diphtheria Antitoxin therapy is of prime impoance. Procaine penicillin 1-2 MU daily for 10 days has adjuvant value and prevents carrier state. 7. Tetanus and gas gangrene Antitoxin and other measures are more impoant; PnG 6-12 MU I day is used to kill the causative organism and has adjuvant value. 8. Penicillin G is the drug of choice for rare infections like anthrax, actinomycosis, trench mouth, rat bite fever and those caused by Listeria monocytogenes, Pasteurella multocida. 9. Prophylactic uses (a) Rheumatic fever: Low concentrations of penicillin prevent colonization by streptococci responsible for rheumatic fever. Benzathine penicillin 1 .2 MU every 4 weeks till 18 years of age or 5 years after an attack, whichever is more. (b) Bacterial endocarditis: Dental extractions, endoscopies, catheterization, etc. cause bacteremia which in patients with valvular defects can cause endocarditis. PnG can afford protection, but amoxicillin is preferred now. (c) Agranulocytosis patients: Penicillin may be used alone or in combination with an aminoglycoside antibiotic to prevent respiratory and other acute infections. ESSENTIALS OF MEDICAL PHARMACOLOGY K.D.TRIPATHI SIXTH EDITION PAGE NO:698,699