There is no as such medicines for opening mouth all u have to stop cause of it like if u taking pan tobacco or smoke kindly stop this the root cause once stop you automatically can practice to move ur jaw so muscles come out from spasm .put ur fingers 2-3 and increase this finger to 3-4 if pain kindly take muscle relaxers tab

Hello chandran , it's better to visit a doctor to confirm your findings and then he or she can prescribe you appropriate medicine which will improve your mouth opening and pain. In certain conditions just few antibiotics are sufficient in rare cases you may need other drugs to increase your mouth opening that can be decided only by examining your tooth and degree or pain. So please avoid self medication