Do you know? Cough syrup contains dextromethorphan, which is a psychotropic substance that can cause dependency and withdrawal symptoms. Higher doses of DXM have dissociative effects and very high doses can cause hallucinations. Short-term DXM side effects include dizziness, nausea/vomiting, feelings of dissociation from the body, hallucinations, impaired judgment, disorientation and loss of motor coordination. These effects can last for up to six hours. Always take your meds exactly as your doctor prescribes.