Prostatism and erectile dysfunction (ED) are highly prevalent and closely comorbid. Prescription treatments are limitingly expensive but robust in mechanisms of action (MoA). Nutritional supplements (NS) are low-cost but inadequately supported by evidence. Do any NS use robust MoA? Could their efficacy be amplified via dosing, concentration of active principles, and/or use in combination? The goal is to develop an effective NS for prostatism and ED using the MoA of prescription treatments. Literature reviews were conducted on dietary supplements for prostatism or ED and MoA of relevant drugs. The most promising NS employing these MoA were chosen. A pilot study of a prototype combination was conducted. A protocol was created for an adequate dose-response trial to test the NS combination in men with ED and prostatism. The main measures were response rates, International Prostate Symptom Score, and International Index of Erectile Function. For drugs, the MoAs best proven for prostatism and ED were nitric oxide augmentation, mild androgen inhibition, and anti-inflammatory effects. The following NS best simulate these MoA and are best supported for efficacy; for prostatism: beta sitosterol; for ED: panax ginseng, arginine, and citrulline. Pilot clinical data provided support. A plan for a formal dose-response clinical trial was approved by a central institutional review board. NS using effective MoA might suffice for prostatism and ED. Pilot testing of a combination NS with the best-supported MoA supported further development. A dose-response trial should be conducted using adequate doses of L-citrulline, beta-sitosterol, ginseng, and vitamin D3.