Eight consecutive patients with histories of penicillin G anaphylaxis have been given the usual dose (1 gm) of sodium methicillin without any reaction. Immediate type skin testing to aqueous potassium penicillin G has been positive in each patient, and sodium methicillin (Staphcillin) tests have been negative except for a nonspecific irritant reaction when high concentrations have been used.
Luton EF. Methicillin Tolerance After Penicillin G Anaphylaxis. JAMA. 1964;190(1):39-40. doi:10.1001/jama.1964.03070140045005