Vaginal pessaries. Canesten pessaries used to treat thrush (candidiasis), a common fungal infection of the vagina. Canesten pessaries contain the anti-fungal drug clotrimazole. The pessaries are inserted high into the vagina, including at times of menstruation. Side effects of this drug include local irritation, bleeding and fluid accumulation (oedema). Thrush is caused by Candida albicans, a yeast-like fungus found on the skin and in the respiratory, alimentary & genital tracts. Normally harmless, it can become pathogenic when the body's resistance is weakened.