I have had lower back pain for several months, mainly on the left side?

Having suffered for such a long time, I'd recommend you visit a physiotherapist (www.csp.org.uk) as a simple injury or strain should have cleared up by now.

By all means use Arnica Gel or Devils Claw for some relief in the meantime but chronic pain can be quite depressing, so do address the root cause.

Best wishes
Earle