… aciclovir in severely immune-compromised individuals may result in the selection of virus strains with reduced sensitivity, which may not respond to continued aciclovir treatment (See section 5.1).
Aciclovir should also be used with caution in patients with underlying neurological abnormalities, severe hepatic or electrolyte abnormalities or significant hypoxia.
Patients with rare hereditary problems of galactose intolerance, the Lapp lactase deficiency or glucose-galactose malabsorption should not take this medicine.