CoQ10 is well-tolerated, and no serious adverse effects have been associated with its use. To date, these include epigastric discomfort (0.39%), appetite suppression (0.23%), nausea (0.16%) and diarrhea (0.12%).

A large 3-month study conducted in 173 Italian centers involving 2664 patients resulted in 1.5% of the patients reporting minor adverse effects. The daily dosage of CoQ10 was 50-150 mg orally, with the majority of patients (78%) receiving 100 mg/day. Clinical and laboratory parameters were evaluated at the entry into the study and on day 90; the assessment of clinical signs and symptoms was made using from two-to seven-point scales.

The study results show a low incidence of adverse effects: "38 adverse effects were reported in 36 patients (1.5%) of which 22 events were considered as correlated to the test treatment. After three months of test treatment the proportions of patients with improvement in clinical signs and symptoms were as follows: cyanosis 78.1%, oedema 78.6%, pulmonary rales 77.8%, enlargement of liver area 49.3%, jugular reflux 71.81%, dyspnoea 52.7%, palpitations 75.4%, sweating 79.8%, subjective arrhythmia 63.4%, insomnia 662.8%, vertigo 73.1% and nocturia 53.6%. Moreover we observed a contemporary improvement of at least three symptoms in 54% of patients; this could be interpreted as an index of improved quality of life." (Baggio E, Gandini R, Plancher AC, Passeri M, Carmosino G. Mol Aspects Med 1994;15 Suppl:s287-94)

According to some of the reports, taking CoQ10 late night may cause insomnia for some people.

These adverse effects may be minimized if daily doses higher than 100 mg are divided into two or three daily doses. (Hendler SS, Rorvik DR, eds. PDR for Nutritional Supplements. Montvale: Medical Economics Company, Inc; 2001.)

Because the safety of CoQ10 during pregnancy hasn't been established, pregnant and lactating women should consult with their physicians before taking this nutrient.