Cephalexin should be used group A streptococcal pharyngitis in those with a penicillin allergy other than anaphylaxis.
Group A Streptococcus is a relatively common cause of pharyngitis in children. Penicillin and Amoxicillin are first line. Cephalexin should be used in those with a non-anaphylactic penicillin allergy and clindamycin should be used for those with anaphylaxis to penicillins given rare cross reactivity. Macrolides including azithromycin, clarithromycin and erythromycin are alternatives.
References:
Shulman ST, Bisno AL, Clegg HW, Gerber MA, Kaplan EL, Lee G, Martin JM, Van Beneden C; Infectious Diseases Society of America. Clinical practice guideline for the diagnosis and management of group A streptococcal pharyngitis: 2012 update by the Infectious Diseases Society of America. Clin Infect Dis. 2012 Nov 15;55(10):e86-102. PMID:22965026.
American Academy of Pediatrics. Group A Streptococcal infections. In: Kimberlin DW, Brady MT, Jackson MA, Long SS, eds. Red Book: 2018 Report of the Committee on Infectious Diseases. American Academy of Pediatrics; 2018;