Welcome Random Man. We all feel your pain here. Here are some tips that may help you, although some easy searching can give you a wide range of tips.
I currently use Solgar Niacin 250mg, you can get this off Amazon or iHerb, whichever is cheaper and convenient for you. I take it an hour or so before I plan to have sex or masturbate, and usually I would get a flush for 30 minutes -- red skin and intense itchiness; I recommend hydrating your skin with oils or just cream to prevent any cracking, if you subconsciously start scratching during the flush.
If possible, and I mean... I highly highly suggest. Do not orgasm. Have sex until the peak, and then stop. It isn't worth it in the long run. Just finish your partner off... only because Niacin does not (for me at least) prevent all symptoms.
Good luck.