The following are studies on supplements which have been shown to have some effect on penis girth.
L Dopa
A double blind study with 51 participants
100mg Levodopa was given to healthy, young men and women in order to assess its effects on sexuality and libido. T-reflex (a measure of physical reaction to eroticism) was increased significantly in the levodopa group, and seemed to affect men more. Sexual desire was not significantly affected by treatment
The effects on penile girth were not significantly effected by treatment relative to placebo.
Yohimbine
Non-controlled Trial with 18 participants
An increase in penile girth has been noted (similar to L-DOPA) which was not to a large degree, only detectable when dividing the group into responders and nonresponders.
Study Summary
Yohimbine at 5.4 or 10.8mg in two divided doses failed to influence testosterone levels. Penile girth was increased only when divided into responders.
Saffron
Randomized controlled trial with 20 participants
Supplementation of a high dose of saffron (200mg from the stigma; 19.7mg/g and 0.25mg/g crocins and safranal respectively) in men with erectile dysfunction noted that supplementation was able to increase penile tumescence (base and tip) and improved erectile function as assessed by the IIEF-15 relative to baseline.
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Panax Ginseng
Double blind study with 45 participants
An improvement in erectile dysfunction is seen with 3g of Korean Red Ginseng (fermented panax ginseng, regular panax ginseng is not as well tested) which is thought to be secondary to anti-fatigue effects and improved blood flow.
900mg of Korean Red Ginseng taken thrice daily (2,700mg total) for 8 weeks showed improvements on subjective ratings of erection quality in persons with diagnosed erectile dysfunction.