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Aciclovir works by attacking the herpes simplex virus, of which there are two types:
Type 1 herpes simplex virus is the usual cause of cold sores around the mouth. It also causes up to half of the cases of genital herpes.
Type 2 herpes simplex virus usually only causes genital herpes. It can sometimes cause cold sores.
Aciclovir does not kill the herpes simplex virus, but it does prevent it from multiplying. This means that, although it may have little effect on existing blisters, it can prevent them from getting worse. If you use aciclovir cream as soon as your symptoms start then it will reduce the severity and duration of the symptoms. Aciclovir cream is available on prescription for the treatment of cold sores and genital herpes infections.