I realize this study was done on Rams, but I would be willing to bet it works for humans too

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Title: The effect of intramuscular vitamin C administrations on semen quality in rams.

Personal Authors: Sönmez, M., Demirci, E.


Abstract:

In this study, 6 Ak-Karaman [White Karaman] rams were subjected to semen collection by electroejaculation once every 3 days for 1 month. Blood samples were also taken after semen collection. 20 mg/kg vitamin C was then injected intramuscularly for 30 days, after which semen and blood samples were again obtained at the same intervals to determine vitamin C levels. It was shown that the pre- and post-vitamin C administration values for average semen volume, sperm concentration and sperm motility were 1.02±0.02 ml, 2.677±0.055×109 ml and 80.7±0.31% and 1.18±0.02 ml, 3.027.0.04×109 ml and 82.2±0.24%, respectively. Average vitamin C levels in the blood and semen before and after vitamin C administration were 0.427±0.01 and 3.384±0.08 and 1.596±0.03 and 3.915±0.07 mg/100 ml, respectively. In conclusion, intramuscular administration of vitamin C in rams increases the concentration of vitamin C in the blood and semen and increases semen volume and sperm concentration, and can therefore improve the semen quality during the breeding season.