Penile bruising is a rare occurrence, but when it does happen, it is important to rule out a number of possibilities that could be causing this irregularity. Generally, the underlying cause of any form of bruising is micro-vascular trauma. Ripping and tearing of veins or capillaries can lead to blood pooling in tissue and lead to bruising. Unfortunately, the penis is an organ that is not free of suffering this same fate.
Some of the more likely causes of a bruise on penis or micro-vascular trauma can occur anytime a male gets dressed or utilizes the bathroom. Zipper injuries from males wearing shorts or jeans and inadvertently "tagging" the skin and lead to minor yet potentially unnoticed pain if it was minor pinching. This is even more likely when the male is uncircumcised.
Another potential cause of penile bruising that may be painless would be due to excessive force during masturbation activities. When males become aroused, at times there may be excessive force applied to the penis leading to an erection. When lotions or lubes are not used, this force can become magnified when excessive friction can occur.
Although self-injury is common, many sexual intercourse acts can also result in trauma to the penis. During erected states, the penis is more vulnerable to injury due to vascular engorgement and it is at this point that the penis can be seriously injured. If you notice painless bruising after sexual activity, oral intercourse could be a reason for this emergence. The soft tissue of the penis is sensitive to damage from the teeth of a partner and trauma may go unnoticed during sexual intercourse due to heightened sympathetic nervous stimulation.Bruising may not appear until a few days after this sexual intercourse so it may be hard to patients to remember such trauma after some period of time.
Another serious but more obvious reason for penile bruising comes in the form of a penile fracture. Like damage to the soft tissue after oral sex during a penile erection, this trauma is only possible when the penile is fully erect. The injury occurs when partners attempt sex positions where the majority of the weight of the partner is localized to the shaft of the penis. The damage can be localized to the corpus cavernosum, the tissue that is the main component of the shaft of the penis, and blood is unable to escape after a penile erection has occurred. The penis will often be bent to one side and may be extremely painful. Nevertheless, patients may not venture to the ER Department instantaneously due to the embarrassing nature of the injury. In one study, some males went 9 to 13 days after an injury before presenting to the ER. 
All in all, however, most penile bruising is minor in nature and patients should just self-monitor their condition to see that there is symptom resolution. Painless injuries suggest that it is a self-limiting micro-vessel disorder and our coagulation system should kick into action with the discoloration disappearing within 7 to 10 days after initial injury. In the event you are on blood thinners or medications to enhance erections like Viagra, it is important to consult with your urologist to make sure nothing more sinister is occurring under your skin.
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