Type 2 diabetes mellitus (T2DM) is associated with a tumor-promoting metabolic environment, yet epidemiological evidence suggests an inverse association with prostate cancer (PCa), giving rise to the PCa–T2DM paradox. We investigated whether major proposed mechanisms could reproduce the effects of plasma from patients with T2DM in prostate cell models. PC3, LNCaP, and HPEpiC cells were exposed to 10% plasma from 16 patients with T2DM for 24 h. Antidiabetic drugs, testosterone and estradiol, glucose, and methylglyoxal were then evaluated independently. Patient-derived plasma markedly reduced viability to 29.0 ± 3.9% in PC3, 67.4 ± 13.6% in LNCaP, and 53.1 ± 2.0% in HPEpiC cells. None of the tested antidiabetic drugs reproduced this response at clinically relevant concentrations. Hormonal modulation altered macrophage polarization and prostate-cell viability but produced only selective, cell-dependent effects. Increasing glucose concentrations enhanced viability in PC3 and LNCaP cells, whereas methylglyoxal reduced viability only in HPEpiC cells at the highest concentrations tested. Overall, none of the major isolated pharmacological, hormonal, glycemic, or glycation-related mechanisms reproduced the plasma-associated phenotype, suggesting that it may arise from the integrated influence of multiple circulating components and signaling pathways.