A middle aged man presented with pain in back, lack of interest in recreational activities, low mood, lethargy, decreased sleep and appetite for two months. There was no history suggestive of delusions or hallucinations. He did not suffer from any chronic medical illness. There was no family history of psychiatric illness. Routine investigations including haemogram, renal function tests, liver function tests electrocadiogram did not reveal any abnormality. This patient should be treated with -