A potentially dangerous side effect of ubiquinone is an allergic reaction that may include dizziness, breathing difficulty, rash, itching, and swelling of the face, tongue or throat, states WebMD. This type of reaction is rare but requires immediate medical attention if it occurs.
Ubiquinone, also known as coenzyme Q10, does not produce many side effects, according to WebMD. Gastrointestinal issues, such as nausea, stomach upset, loss of appetite and diarrhea, occur in rare instances. Sleeping difficulty is another rare side effect, notes Drugs.com.
Some forms of this supplement contain sugar, alcohol or aspartame, which may cause unwanted effects in people with certain medical conditions, such as diabetes and phenylketonuria, warns WebMD.