PureSorb-Qsup(TM)40 (water-soluble type CoQsub(10) powder, CoQsub(10) content is 40 w/w%; hereinafter referred to as P40) is reported in the single-dose human and rat studies to have a greater absorption rate and absorbed volume of CoQsub(10) even taken postprandially, than those of regular CoQsub(10), which is lipid-soluble and generally taken in the form of soft-gel capsules. Thus, it was anticipated that the serum CoQsub(10) level might be higher with P40 tablets than with soft-gel capsules, even for the same dose of CoQsub(10). In the present study, in order to confirm the safety and measure the serum CoQsub(10) level for the case of an excessive dose of P40, a double-blinded Placebo controlled comparative study was conducted on 46 healthy volunteers and they were randomly divided into two groups. The P40 tablets or placebo were repeatedly taken by the volunteers. As the result of the study, for the group of taking 2,250 mg/d. of P40 (that is, 900 mg/d of CoQsub(10)) for 4 consecutive wk, the serum CoQsub(10) level peaked at 2 wk after the start of intake at 8.79 +- 3.34 microg/mL, and at 4 wk, it was at the level of 8.33 +- 4.04 microg/mL. At 2 wk from withdrawal of intake, the serum CoQsub(10) level decreased to 1.30 +- 0.49 microg/mL. The serum CoQsub(10) levels at these three points were significantly higher than those of the first day of intake and the Placebo group, which had no significant change throughout the study.