There are two major factors that lead to deficiency of CoQ10 in humans: reduced biosynthesis, and increased use by the body. Biosynthesis is the major source of CoQ10. Biosynthesis requires at least 12 genes, and mutations in many of them cause CoQ deficiency. CoQ10 levels also may be affected by other genetic defects (such as mutations of mitochondrial DNA, ETFDH, APTX, FXN, and BRAF, genes that are not directly related to the CoQ10 biosynthetic process). The role of statins in deficiencies is controversial.

Some adverse effects, largely gastrointestinal, are reported with very high intakes. The observed safe level (OSL) risk assessment method indicated that the evidence of safety is strong at intakes up to 1200 mg/day, and this level is identified as the OSL.
Although CoQ10 may be measured in blood plasma, these measurements reflect dietary intake rather than tissue status. Currently, most clinical centers measure CoQ10 levels in cultured skin fibroblasts, muscle biopsies, and blood mononuclear cells.Culture fibroblasts can be used also to evaluate the rate of endogenous CoQ10 biosynthesis, by measuring the uptake of 14C-labelled p-hydroxybenzoate.

While statins may reduce coenzyme Q10 in the blood it is unclear if they reduce coenzyme Q10 in muscle.Evidence does not support that supplementation improves side effects from statins.
CoQ10 is not approved by the U.S. Food and Drug Administration (FDA) for the treatment of any medical condition.It is sold as a dietary supplement. In the U.S., supplements are not regulated as drugs, but as foods. How CoQ10 is manufactured is not regulated and different batches and brands may vary significantly.

A 2004 laboratory analysis by ConsumerLab.com of CoQ10 supplements on the market found that some did not contain the quantity identified on the product label. Amounts varied from "no detectable CoQ10", to 75% of stated dose, and up to a 75% excess.
Generally, CoQ10 is well tolerated. The most common side effects are gastrointestinal symptoms (nausea, vomiting, appetite suppression, and abdominal pain), rashes, and headaches.

While there is no established ideal dosage of CoQ10, a typical daily dose is 100–200 milligrams. Different brands may have varying ingredients and strengths.