Mantra for pain relief in Hinduism is dedicated to Dhanwantari, the God of medicine and cure. The mantra will only be effective when a person has devotion and is able to shed all kinds of negativity including hatred, jealousy and anger. Here is the mantra for pain relief.
ॐ शंखं चक्रं
स्वरूपाया श्री
श्री औषधचक् धन्वंतरये
नमः
Om Shanka chakra swaroopaya sri sri oushadhachakra dhanvantareye Namah
Om Shanka chakra swaroopaya sri sri oushadhachakra dhanvantareye Namah
The mantra should be chanted early morning just before sunrise.
The person chanting the mantra should sit facing east or north or northeast.
The mantra should be chanted 51 times.
The person should plant herbs like tulsi, lemon grass, mint etc and take care of it.
It is also highly meritorious to feed fishes.