It may cause serious health problems when overdosed. Signs and symptoms of adverse effects may include any or several of the following: convulsions, respiratory depression (slow or stopped breathing), dilated eye pupils, nystagmus (rapid side-to-side eye movements), erythema (flushed skin), gastrointestinal constipation, nausea, vomiting, paralytic ileus, tachycardia (rapid pulse), drowsiness and hallucinations. Symptoms of toxicity may take up to 12 hours to appear.