A cough lasting 20 days with a suspected chest infection requires a professional medical evaluation rather than self-medication. Since over-the-counter cough syrups have not provided relief, you may need targeted prescription treatments such as antibiotics (if it is a bacterial infection) or inhalers. These can only be safely prescribed after a doctor listens to your lungs with a stethoscope and potentially orders a chest X-ray. Please do not start any new medications without a physical examination. I strongly advise you to visit a doctor or a local clinic immediately for an accurate diagnosis and the correct prescription.