The common causes of left arm pain and weakness are as follows: 1. Cervical spondylosis: This is the most common condition, in which neurogenic cervical spondylosis can involve the left upper limb. When the protruding cervical disc compresses the nerve root, the patient can experience radiating pain in the upper limb, which can radiate from the neck to the shoulder and then to the upper arm, elbow and hand. If the pressure on the nerve root is more serious, it will also affect the muscle strength of the upper limb, making the patient’s left arm weak and unstable. 2. Rotator cuff injury: rotator cuff is the power muscle of shoulder joint abduction, when rotator cuff tears occur, the patient can experience pain in the left arm, especially in the left shoulder, with the most obvious pain at night, and at the same time, the patient has weakness in lifting the shoulder joint.