Kratom is well worth trying but it can be slightly addictive. Adding Kava to Kratom has been said to be quite effective, Kava acts as a mild muscle relaxant and Kratom helps control pain. You could look into poppy pod tea but this is also slightly addictive.
Fact of the matter is that you aren't likely to find any strong pain relievers that don't have at least some addictive properties (discluding NSAIDs).
There are some plant extracts that are said to have anti-inflammatory properties but I can't vouch for their effectiveness.
White Willow Bark
Curcumin
Bromelain
Devil's Claw
Boswellia