Purpura simplex is extremely common. The cause is not known. Some doctors think it may be a feature of many disorders, but others think it is a normal variation.
Aspirin and drugs that prevent blood clots (for example, warfarin), low body weight, prior use of corticosteroid drugs, and sun exposure may increase the risk of purpura simplex.
Purpura simplex usually affects women. Bruises develop on the thighs, buttocks, and upper arms in people without known injuries. People may have family members who also bruise easily. Although doctors often do tests of the blood clotting system to look for clotting abnormalities, no abnormalities are found (see also Bruising and Bleeding).
This condition is not serious, and no treatment is needed. However, sometimes doctors recommend that people avoid taking aspirin and aspirin-containing drugs.
Bleeding under the skin may also be more common among older people (a condition called senile purpura). Senile purpura commonly causes bruising on the hands and forearms.