Aciclovir is an antiviral medicine used to prevent or treat infections from the herpes simplex virus (HSV). It\'s commonly found in the over the counter cold sore cream Zovirax. It works by stopping the growth of the virus and suppresses any unwanted symptoms. Aciclovir can also be used as a means to prevent recurrent outbreaks and is one of the more popular treatments used in the UK for genital herpes.  Genital Herpes is caused by HSV, which itself is highly contagious, and can be spread by any direct contact with the affected areas. Using a barrier method is the only way to fiully prevent transmission of genital herpes between sexual partners. Two different strains of the HSV virus exist. Aciclovir will effectively help to treat both strains of the virus. Aciclovir targets the enzyme that replicates and grows the virus. Within this time the body\'s own immunity will help to fight the virus and end the outbreak. If Aciclovir is taken as soon as symptoms appear, the outbreak will often disappear within one to two days. Recurrent outbreaks can usually be detected by a tingling or itching sensation before blisters ever appear. Aciclovir, if taken as soon as you notice these signs, can prevent blisters from ever appearing during that outbreak.  After using Aciclovir to treat an outbreak, patients should wait until the affected area has healed before resuming sexual activity. Only then will it not be contagious.