This natural source of caffeine and other alkaloids has been shown to improve erectile function in middle-aged men when used in combination with other ingredients found in Alpha Male Plus. Moreover, the caffeine content in guarana has been shown to improve sexual function in men using PDE-5 inhibitors, when used in combination with Citrulline and Resveratrol (which are both found in Alpha Male Plus).
S. Nguyen, J. Rajfer, and M. Shaheen, “Safety and efficacy of daily Revactin® in men with erectile dysfunction: A 3-month pilot study,” Transl. Androl. Urol., 2018.
M. Shirai et al., “Oral Testofen, L-Citrulline, Resveratrol, and Caffeine Supplement Drink Improves Sexual Function in Men with Phosphodiesterase 5 Inhibitors: Randomized, Double-Blind, Placebo-Controlled Crossover Pilot Study,” World J. Mens. Health, 2020.