Detumescence is the process during which blood leaves the erect penis, returning it to a flaccid state. It is often a stage when the penis is subsiding from sexual tension, swelling, and sexual arousal. This can happen when an erection goes down or after a sexual encounter.
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Detumescence is when swelling or an erection subsides. This can be a natural process after ejaculation or it can occur when there is a performance anxiety issue or difficulty maintaining an erection.