ADVERSE REACTIONS Guaifenesin is well tolerated and has a wide margin of safety. Nausea and vomiting are the side effects that occur most commonly. Other reported adverse reactions have included dizziness, headache, and rash (including urticaria). Codeine: Nausea, vomiting, constipation, drowsiness, and miosis have been reported. Higher doses may induce euphoria, light headedness, dizziness, drowsiness, and depression of respiration. Pruritus and skin rashes have been rare.