A sustained release formulation as a unit dose contains 100 mg-1000 mg of Acetaminophen and 15 mg-150 mg of tramadol hydrochloride, which comprises of 1) an immediate release portion comprising of 25%-75% of the total effective amount of drug in the dosage form and 2) a sustained release portion comprising of a) 25%-75% of the total effective amount of drugs in the dosage form; b) 6%-50% of gelling polymers of the total formulation, and c) optionally an enteric coating at a level of 5%-40% of the total formulation. The set forth formulation dissolves 25%-60% of the total drug in the first hour, 50%-90% of the total drug in the first four hours and not less than 80% of the total drug in the first 12 hours using USP dissolution method II at 50 rpm.