Dose of Aciclovir must be adjusted in patients with impaired renal function in order to avoid accumulation of Aciclovir in the body. Concomitant use of other toxic drugs, pre-existing renal disease and dehydration increase the risk of further renal impairment. Contact with unprotected skin should be avoided. Special care should be taken in pregnancy and lactation. Aciclovir should not be used during pregnancy unless the potential benefits to the patient outweigh the potential risk to the foetus.