CoQ10 Side Effects
Coenzyme Q10 is extremely safe, but we don’t know about its effects on pregnant or lactating women, or longer dosages on small children. Until it has been tested, I would not recommend it to these groups unless a patient has an urgent disease, such as a heart problem, that warrants its use (and then, only with the supervision of a doctor).

CoQ10 may hinder the action of the blood-thinning drug Coumadin. If you’re taking that pharmaceutical, let your doctor know that you plan on supplementing with CoQ10 to ensure that your blood-clotting ability can be closely monitored. Personally, I have never seen a bad interaction between Coumadin and CoQ10, however some problems have been reported in the medical literature. Additionally, I have noted some patients experience insomnia when CoQ10 is taken in the evening, presumably because of its energy-enhancing enhancing effects.