The effect of testosterone on the metabolic activity of hormone-independent [PC-3] and hormone-dependent [LNCAP] cancer cells was investigated in vitro. Additionally, the impact of testosterone nanoemulsion [nanocare®] on cell viability was accessed by flow cytometry. Despite the dependency of the normal prostate and of most prostatic cancers upon androgens, the obtained results indicate that, contrary to prevailing opinion, the supplementation of testosterone with higher doses in nanoemulsion was able to lower the metabolic activity and viability of prostate cancer cells. We conclude that the growth of hormone-independent and hormone-dependent prostate cancer cells was reduced by the exposure of a nanoemulsion of bioidentical testostosterone in vitro. To the best of our knowledge, this is the first time that the potential effect of a testosterone nanoemulsion on the metabolic activity of prostate cancer cells has been shown. Such tests suggest that the growth of hormone-independent and hormone-dependent prostate cancer cells was reduced by the administration of bioidentical testostosterone, and this might be an interesting strategy for prostate cancer treatment in diagnosed patients.