Side effects of taking dextromethorphan include slight nausea, vomiting and drowsiness, reports WebMD. Other less serious side effects include constipation, restlessness and nervousness, according to Everyday Health. Serious side effects that warrant an immediate visit to the doctor include skin rashes, blackouts, hypertension, double vision and respiratory depression.
Some dextromethorphan side effects may not need medical attention as they are a result of the body getting used to the medication, explains Drugs.com. Dextromethorphan may affect the nervous system at normal doses and cause mild dizziness and drowsiness. Gastrointestinal side effects of dextromethorphan include stomach upset, constipation and diarrhea.
An overdose of dextromethorphan may cause severe side effects, and the patient may require immediate medical attention, notes Drugs.com. Some of the side effects of an overdose include difficulty urinating, severe nausea and vomiting, unsteady walking, confusion and slowed breathing. Side effects on the nervous system due to high dosages of dextromethorphan are unusual hyperactivity, euphoria and hallucinations.
Allergic reactions to dextromethorphan are rare but include skin rashes, severe dizziness and trouble breathing, reports WebMD. Dextromethorphan may also cause side effects when combined with certain medications such as manerix, selegiline, phenelzine and rasagiline, according to Everyday Health. People should not take dextromethorphan with grapefruit, as this may increase the amount of dextromethorphan in their system.