Canesten 200mg pessaries contain the active ingredient clotrimazole, which is an antifungal medicine used to treat infections with fungi and yeasts, such as Candida Fungi (thrush infection). Clotrimazole kills the Candida fungi causing the thrush infection by interfering with their cell membranes. 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. Clotrimazole makes holes appear in these membranes, which kills the fungi and clears up the infection. The pessaries are inserted into the vagina to allow the clotrimazole to act directly in the area of infection.