What Clotrimazole is and what it is used for? The active substance is clotrimazole. Clotrimazole belongs to a group of medicines called imidazoles and is an antifungal agent which fights the cause of fungal skin infections. Clotrimazole is used to treat fungal skin infections such as ringworm, athlete’s foot, fungal nappy rash and fungal sweat rash. It is also used to relieve irritation of the vulva (external thrush) or the end of the penis, which may be associated with thrush. When Clotrimazole is applied to the infected area of skin it works by destroying the fungus or yeast which has caused the infection.