Lamisil contain the active ingredient terbinafine hydrochloride, which is an antifungal medicine used to treat infections caused by fungi. (NB. Terbinafine is also available without a brand name, ie as the generic medcine.) Terbinafine kills fungi by interfering with their cell membranes. It works by stopping fungi from producing a substance called ergosterol, which is an essential component of fungal cell membranes. The disruption in production of ergosterol disrupts the cell membrane and causes holes to appear. The cell membranes of fungi are vital for their survival. They keep unwanted substances from entering the cells and stop the contents of the cells from leaking out. As terbinafine causes holes to appear in the cell membranes, essential constituents of the fungal cells can leak out. This kills the fungi and hence clears up the infection. Terbinafine is taken by mouth to treat a wide range of fungal skin infections that require oral treatment due to either the severity, site, or extent of the infection. Terbinafine tablets are also used to treat fungal nail infections, which can be difficult to treat topically with creams etc.