According to WebMD, one should consult a doctor if treatment with drugs such as Clotrimazole or Terbinafine does not clear up ringworm within four weeks. Clotrimazole and Terbinafine are available as a topical cream or lotion in both non-prescription and prescription strengths.
Ringworm is caused by a fungal infection that does not normally penetrate below the top layer of the skin. It often responds to non-prescription drugs such as Lotrimin AF and Lamisil AT. The Mayo Clinic states that several weeks of oral prescription drugs may be necessary in more severe cases. Patients with compromised immune systems can have problems fighting the infection.