The use of coenzyme Q10 as a treatment for cancer in humans has been investigated in only a limited manner. The studies that have been published consist of randomized controlled trials, anecdotal reports, case reports, case series, and uncontrolled clinical studies.[1,2,3,4,5,6,7,8,9,10,11,12]
In view of the promising results from animal studies, coenzyme Q10 was tested as a protective agent against the cardiac toxicity observed in cancer patients treated with the anthracycline drug doxorubicin. It has been postulated that doxorubicin interferes with energy-generating biochemical reactions that involve coenzyme Q10 in heart muscle mitochondria and that this interference can be overcome by coenzyme Q10 supplementation.[2,13,14] Studies with adults and children, including the aforementioned randomized trial, have confirmed the decrease in cardiac toxicity observed in animal studies.[1,2,3,7] A randomized trial  of 20 patients tested the ability of coenzyme Q10 to reduce cardiotoxicity caused by anthracycline drugs.
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A larger randomized, placebo-controlled trial of 236 breast cancer patients concluded that coenzyme Q10 at a daily dose of 300 mg combined with 300 IU of vitamin E, divided into three doses, did not improve fatigue levels or quality of life after 24 weeks of supplementation.
The potential of coenzyme Q10 as an adjuvant therapy for cancer has also been explored. In view of observations that blood levels of coenzyme Q10 are frequently reduced in cancer patients,[6,10,11,15,16] supplementation with this compound has been tested in patients undergoing conventional treatment. An open-label (nonblinded), uncontrolled clinical study in Denmark followed 32 breast cancer patients for 18 months. The disease in these patients had spread to the axillary lymph nodes, and an unreported number had distant metastases. The patients received antioxidant supplementation (vitamin C, vitamin E, and beta carotene), other vitamins and trace minerals, essential fatty acids, and coenzyme Q10 (at a dose of 90 mg/day), in addition to standard therapy (surgery, radiation therapy, and chemotherapy, with or without tamoxifen). The patients were seen every 3 months to monitor disease status (progressive disease or recurrence), and, if there was a suspicion of recurrence, mammography, bone scan, x-ray, or biopsy was performed. The survival rate for the study period was 100% (4 deaths were expected). Six patients were reported to show some evidence of remission; however, incomplete clinical data were provided, and information suggestive of remission was presented for only 3 of the 6 patients. None of the 6 patients had evidence of further metastases. For all 32 patients, decreased use of painkillers, improved quality of life, and an absence of weight loss were reported. Whether painkiller use and quality of life were measured objectively (e.g., from pharmacy records and validated questionnaires, respectively) or subjectively (from patient self-reports) was not specified.