The most common cause of low back pain on the left side is a muscle strain ... experience a stabbing pain, or have muscle spasms, says Michael Rosner, MD, a neurosurgeon at George Washington ...

These spasms, often felt in areas like the neck, legs, or back ... and involuntary muscle contractions Certain medications can potentially trigger muscle spasms as a side effect.

Neck pain on the left side usually results from a benign cause, like muscle strain from sleeping in an awkward position. Sometimes, a more serious cause may be a tumor or rheumatoid arthritis.