How can i relieve pain in my back left molar when i bite down?

Short term relief: Brief relief with over the counter Acetaminophen or Ibuprofen but if it hurts when you bite down, time to talk with your dentist about your unhappy molar [possible cavity or worse...]. Hope you're feeling better soon.

See a Dentist: There are three main causes of biting pain in a molar: cavity, infection, or bite problem. Unfortunately, there's nothing you can personally do to correct these problems. Obviously, a soft diet and avoiding pressure on that tooth is a common sense approach to keeping you comfortable. But, that won't fix the problem. You really need to see a dentist before this issue gets worse and more costly.