XYL‑Q‑10 is generally well tolerated, but in some individuals it may cause mild side effects such as nausea, stomach upset, diarrhoea, headache, heartburn, or a fishy aftertaste. These effects are usually temporary and tend to resolve over time. If any side effects persist or worsen, it is important to consult a doctor. Response to the supplement can vary depending on individual tolerance, underlying medical conditions, and concurrent medications.