I think the main health concerns are mostly in regard to techniques which strive to prevent the normal ejaculation of semen during sexual climax and orgasm by forcibly attempting to retain it at that time with specific methods of muscle contraction and mental visualization. This is oftentimes much like trying to prevent urination when the urinary bladder is completely full, and the immediate need to empty the bladder is urgent, or like trying to retain a bowel movement that demands to be evacuated IMMEDIATELY!
As such, if semen retention techniques are practiced too often or too strenuously, there is likely to be some harmful effect or outright injury to the prostate gland and urethra, and perhaps other associated organs, as a result. But here again, it is primarily the overzealous and overstrenuous use of such methods which causes harm, not usually a more normal and moderate practice of these techniques.
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