Your blood vessels widen when their lining releases nitric oxide, and the body makes it from arginine. Citrulline raises the arginine in your blood more reliably than arginine itself does ¹. It does the most for people whose vessels are already stiffening; a healthy young adult has plenty of raw material and won't feel a thing.

Blood pressure. Pooled across eight trials, three to six grams a day drops resting systolic pressure about four points, most in the people who already ran high ². You won't feel four points; at scale it's fewer strokes and heart attacks.

Mild erectile dysfunction. In men whose erections still mostly work but less reliably, half moved back to normal hardness on citrulline versus about one in twelve on placebo ³. The dose was a gram and a half a day.

The gym. Real but small: a bit more of the back-half of a hard set, and the work feels easier afterward ^{4 5}. On top of creatine and caffeine this is one extra rep, never a new training stimulus.

Buy plain L-citrulline powder in bulk. Skip arginine, the "nitric-oxide complex," and the one-gram capsule buried in a blend.

The vascular and erection effects accrue over a week or two of daily dosing and fade if you stop; the workout effect is same-session. The taste is mildly tart, and any flavoured drink hides it.

Inside a week, if you train, the pump is a touch bigger and the second-to-last set goes a rep further ⁶. Inside three to six weeks, the home cuff reads two to five points lower than baseline ². Inside a month, for the mild-ED responder, morning erections come back more often than not ³; for the other half they don't, and the honest move is to stop and try something else. What you never get is a transformed day or a different body. It's a cheap, narrow, daily win.