Can a skin ointment cause dark patches on the skin?

Q: When I was in the 11th standard, I got backache for which the doctor prescribed me medicines with 10 days bed rest. He gave me an ointment Volini. After using this everything was fine. After sometime I got a black patch on backside where I used to put Volini. Please help me with this?

A:Volini Gel contains several ingredients including diclofenac and methyl salicylate. You could be allergic to either one or both. Hence you must never use any ointment/cream/gel containing these medicines.