Limited research on the safety and efficacy of CoQ10 signifies that not all side effects are known. So far, major side effects of oral coenzyme Q10 supplementation have not been reported. There is no data on the side effects of intramuscular or intravenous injection of CoQ10.
Recorded side effects of oral CoQ10 supplementation in adults include rash, epigastric pain, nausea, headache, fatigue, heartburn, increased involuntary movements, anorexia, vomiting, diarrhea, pain in the upper abdomen, dizziness, irritability, sensitivity to light, and elevated liver enzymes.
In children diagnosed with a mitochondrial disorder, oral CoQ10 supplementation has caused mild side effects, including nausea, diarrhea, heartburn, and less commonly reported are headache, dizziness, irritability, and agitation.
There is no data on the short-term or long-term side effects associated with intravenous and intramuscular injection of CoQ10 in humans. There is little data on the long-term effects of oral CoQ10 supplementation. A review of clinical studies reveals that no serious or frequent side effects were observed in adults with oral administration of CoQ10 when taking 200 mg/day for up to 12 months or 100 mg/day for up to 6 years. However, the review also notes that these studies were not designed to examine side effects and therefore suggests that monitoring side effects was not a priority during these studies. One study saw impairments in cognitive and sensory function in mice after prolonged administration of high levels of CoQ10.
The literature is contradictory as to whether coenzyme Q10 lowers blood pressure. In some clinical trials, coenzyme Q10 lowered both systolic and diastolic blood pressure levels. Coenzyme Q10 supplementation is contraindicated for patients who are taking anti‐hypertensive medication. Patients with low blood pressure should use precaution when taking CoQ10.