Background and aim: Herpes simplex virus is one of the most important human pathogenic viruses that may lead to oral herpes, keratoconjunctivitis and even encephalitis. A number of enzymes of the virus such as DNA polymerase can be targeted antiviral drugs. Acyclovir is used to treat infections of the virus, today due to drug resistance, need to do more research on finding new drugs, especially herbal medicines has increased. This study aimed to investigate the antiviral effect of methanol extract of Chelidonium majus compared with acyclovir against the virus in HeLa cell culture.

Methods: In this experimental study, the toxicity of Chelidonium majus L. methanol extract and acyclovir on HeLa cell was determined with both MTT and Trypan blue methods. The antiviral effect of Chelidonium majus L. extract and acyclovir was evaluated in different concentration (1800- 1700- 1600- 1500- 1400 and 500- 100- 75- 50- 30-10 µg/ml) and also in different times before, after and during of virus adsorption respectively.  The virus titer was measured by tissue culture infectious dose 50 )TCID50(  method. The T-test method was used to comparing the effects of both compounds on virus.

Results: The maximum non-toxic concentration of Chelidonium majus L. extract on HeLa cell was determined 1600 µg/ml that has the maximum inhibitory effect on HSV-1 replication. Acyclovir was shown low toxicity on HeLa cells.The 30 µg/ml concentration of acyclovir was considered for the next steps of the study.The highest inhibitory effect of the extract was observed 1 hour after absorption and the virus replication was suppressed completely by the acyclovir immediately after virus adsorption up to 8 hours after infection.

Conclusion: The Chelidonium majus L. methanol extract has less effect than the acyclovir on inhibition of herpes simplex virus replication in a first few hours of infection. More research is needed to achieve effective compounds with antiviral activity of above extract due to increasing acyclovir resistant strains also its side effects.