Introduction & Objective: The side effects of drug consumption and also HSV resistance to drugs have been the factors attracting the researchers to herbal drugs. The major aim of the present research was in vitro assessment of possible anti-herpetic activity of glycyrrhiza gla-bra (liquorices root) methanolic extract in more details by performing Tissue Culture Infec-tive Dose fifty percent (TCID50) method.

Materials & Methods: In this experimental study Vero cells were incubated with different con-centrations of methanolic extracts of glycyrrhiza glabra .Neutral red assay was performed in order to consider the appropriate concentration of the extract. Pre-treatment of Vero cells with glycyrrhiza glabra extract before viral infection, incubation of HSV-1 with glycyrrhiza glabra extract and treatment of vero cells with extract 1, 4, 8, 12 and 24 hours after viral in-fection, were exerted. TCID50 was performed in order to assess the antiviral activity of the extract. The results were analyzed after performing the experiments at least three times.

 Results: Pre-treatment of Vero cells with methanolic extract for two hours and incubation of virus with extract for one and two hours prior to viral infection resulted in remarkable drop in TCID50 amount by significant difference (P <0.001). Treatment of Vero cells with extract 1, 8 and 12 hours post-infection demonstrated significant changes in TCID50. We observed significant fluctuations and different efficacy of the extract between different incubation time at 1h & 4h, 1h & 24h, 4h & 8h, 4h & 12h, 8h & 24h, 12h & 24h.

Conclusion: The current study resulted in more novel findings of anti herpetic activity of gly-cyrrhiza glabra extract. Time course of extract treatment with virus- infected cells was an ef-ficient factor. In addition, the pretreatment of virus and also Vero cells with glycyrrhiza gla-bra extract in our experiment had important effects on the anti-viral activity of the extract.