This could be oral submucous fibrosis OSMF due to tobacco chewing over years.. if the opening is less than a finger medicine are not of much help.. better consult a dentist for full care regime.

Although your description is not very clear, it seems that you are suffering from Oral Submucous Fibrosis. The first suggestion would be to visit a dentist for a proper clinical examination to identify the grade of the disease at present. Also other factors which can cause reduce mouth opening also need to be evaluated. Medicines will not cure the problem of Oral Submucous Fibrosis. There are various treatment modalities with variable success rates, like intralesional injections and surgical intervention. The options best suited for your case can be explained to you after a proper diagnosis has been made. And it goes without saying that quitting the habit is the most important step. Regards.