As of 2015, no reports exist documenting serious toxicity associated with the use of coenzyme Q10, states WebMD. While CoQ10 is likely safe when taken by healthy individuals in daily amounts up to 3,000 milligrams for up to eight months, high doses over longer periods require caution, states Mayo Clinic.
A small number of patients withdrew from a cardiovascular study on coenzyme Q10 after experiencing nausea, upper-abdominal pain and rashes, reports WebMD. Insomnia has been associated with CoQ10 doses of 100 milligrams per day. Patients taking 300-milligram doses per day for long periods have exhibited liver enzyme level increases, although no liver toxicity was found. Additional reported side effects for CoQ10 include sensitivity to light, indigestion, fatigue and headache.
Coenzyme Q10 can render the anticoagulant drug warfarin less effective, reports WebMD. It can also lower the amount of insulin required by diabetics. Individuals with skin disorders should exercise caution when taking CoQ10 because rash and itching are possible side effects, according to Mayo Clinic. Patients with bleeding disorders, bile duct blockage, low blood pressure and thyroid disorders should likewise be aware that coenzyme Q10 may have adverse affects on these conditions or negative interactions with the medications prescribed to treat them.