Pain referred from the spinal joint is called nociceptive. Nociceptive pain occurs when receptors in and around the joint are stimulated. This sends a message of unpleasant stimuli up the spinal cord to the brain. The brain signals back pain that can go down the leg but doesn't go below the knee.
Neuropathic and nociceptive pain responds to drug treatment differently. Nociceptive pain is sensitive to nonsteroidal antiinflammatory drugs (NSAIDs). Neuropathic pain is more likely to respond to antidepressants and anticonvulsants (seizure medication).
The chemical pathway for the pain is different between nociceptive and neuropathic pain. The mechanism of these drugs is based on these chemical pathways and affects them differently. Finding the source of back and leg pain is important so the right treatment approach can be applied. Sometimes it's a matter of trial and error before the right drug is prescribed for the specific problem.