Aciclovir (ACV), also known as acyclovir, is an antiviral medication. It is primarily used for the treatment of herpes simplex virus infections, chickenpox, and shingles. Other uses include prevention of cytomegalovirus infections following transplant and infections due to Epstein-Barr virus. It is available by mouth and intravenously. Common side effects include nausea and diarrhea. Potentially serious side effects include kidney problems and low platelets. Greater care is recommended in those with poor liver or kidney function. It is generally considered safe for use in pregnancy with no harm having been observed. It appears to be safe during breastfeeding. Aciclovir is a nucleic acid analogue made from guanosine. It works by decreasing the production of the virus's DNA. The discovery of aciclovir was announced in 1977. It is on the World Health Organization's List of Essential Medicines, the most important medications needed in a basic health system. It is available as a generic medication and is marketed under many brand names worldwide. The wholesale cost as of 2014 to 2016 was between US$0.03 and US$0.12 for a typical dose by mouth. The cost of a typical course of treatment in the United States is less than US$25.