Since your pain has not improved after 5 days of Mobiswift D, do not start alternative painkillers without a doctor's evaluation. Left neck and arm pain can indicate a pinched nerve (cervical radiculopathy), which requires different prescription medications like gabapentin or pregabalin rather than standard muscle relaxants. Crucially, left-sided pain can also sometimes be linked to heart-related issues. You need a clinical examination and possibly an ECG or spine MRI to find the exact cause. Please consult your doctor for a proper diagnosis. If you experience chest tightness, shortness of breath, sweating, or dizziness, seek emergency medical care immediately.