I have a small carious cavity and clear opg with constant pain. dentist is confused and thinks its pulpitis but not sure. medicines failed.

Internet diagnosis: These types of situations require a clinical exam and x-rays along with other types of tests IN PERSON and cannot be properly or accurately diagnosed over the internet. Sorry. Perhaps see another dentist for a second opinion Medicines? Was the cavity filled? When or why not? perhaps another tooth or another problem?

Need more info....: If you have constant pain, something is amiss. A clinical examination with a radiograph would go a long way in making the proper diagnosis. But since we don't have that information over the internet, and guessing is the only option in this case, have your dentist check for a fracture of the tooth.

OPG?: Perhaps now would be a good time for evaluation by an Endodontist, a root canal specialist.

See Endodontist: According to your symptoms you have typical inflammation of the nerve and need root canal treatment. You should visit an endodontist for evaluation and treatment. They are experts at diagnosing and relieving tooth pain and use advanced equipment to treat patients efficiently and comfortably.

Pulpitis: Probably is pulpitis. Ask for referral to endodontist for confirmation/treatment.