It depends on how hygienic the person is.Usually nothing.
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You could, but you might get an STD or HIV from doing so.
If you find a man who his healthy, or it is your own semen, it can be good for you.
Semen swallowing three times every week can reduce the risk of cancer and other conditions. - sementherapy.com
If you are a man it replenish what you lose from your own ejaculation.
If it is from you boyfriend or husband, it will bring you closer together.