The side effects of cough syrup vary according to age, gender, medical condition. These can be common as well as severe. Common side effects of oral cough medicines are: Headache, Fever, Nausea, Vomiting, and Drowsiness.Severe Reactions include Tremors, Addiction, Irregular heartbeats, and seizures. In any of the cases, you should consult your pharmacist or medical service provider immediately.