Hematomas can form without any obvious bump or blow when something disrupts your body’s ability to clot blood or maintain strong blood vessel walls. The causes range from common medications and vitamin deficiencies to underlying conditions like liver disease, blood cancers, and clotting disorders. In many cases, the explanation turns out to be straightforward once the right blood work is done.
Medications That Increase Bleeding
The most common reason for unexplained hematomas is medication. Blood thinners are the well-known culprits, but the list extends well beyond them. Over-the-counter pain relievers like aspirin and ibuprofen reduce your blood’s ability to form clots, and people who take them regularly sometimes develop hematomas from pressure or movements so minor they don’t register as injuries. Prescription anti-platelet drugs work similarly by keeping blood cells called platelets from sticking together.
Less obvious offenders include certain antidepressants and antibiotics, both of which can interfere with clotting. Corticosteroids, often prescribed for inflammation or autoimmune conditions, thin the skin over time, which makes underlying blood vessels more vulnerable to breaking. If you’ve recently started a new medication and notice unexplained bruising or hematomas, the timing is worth mentioning to your doctor.
Clotting and Blood Disorders
Your body relies on a chain reaction of clotting proteins and platelets to seal off damaged blood vessels. When any link in that chain is missing or defective, blood can pool under the skin or deep in muscle tissue without any trauma.
Hemophilia is one of the best-known examples. People with hemophilia lack specific clotting proteins, so even the normal wear and tear of daily movement can cause internal bleeding that collects as a hematoma. Von Willebrand disease is actually more common than hemophilia and involves a different clotting protein that helps platelets stick to vessel walls. Both conditions can range from mild (occasional unexplained bruises) to severe (large, deep hematomas or joint bleeds).
Blood cancers like leukemia and lymphoma can also cause spontaneous hematomas. These cancers crowd out normal blood cells in the bone marrow, which means your body produces fewer functional platelets. Without enough platelets, even microscopic blood vessel damage that normally seals itself in seconds can lead to visible bleeding under the skin.
Liver and Kidney Disease
The liver manufactures nearly every clotting protein in your blood, including fibrinogen and factors II, V, VII, IX, X, XI, and XII. When the liver is damaged by cirrhosis, hepatitis, or heavy alcohol use, production of these proteins drops. The result is a bleeding tendency that can show up as easy bruising, nosebleeds, or hematomas that seem to appear out of nowhere.
Kidney disease contributes through a different mechanism. In chronic kidney disease, waste products build up in the blood and interfere with how platelets function. Research published in the Journal of the American Society of Nephrology found that patients with chronic kidney disease consistently show decreased platelet adhesion and prolonged bleeding times. Their platelets are physically present but don’t activate properly, so clots form slowly or incompletely.
Vitamin Deficiencies
Two vitamins play direct roles in preventing unwanted bleeding. Vitamin K is essential for producing several of the clotting proteins your liver makes. Without adequate vitamin K, your blood simply cannot form stable clots. This deficiency is relatively rare in adults who eat a varied diet (leafy greens are a major source), but it can develop in people with digestive conditions that impair nutrient absorption, those on prolonged antibiotic courses that disrupt gut bacteria, or people with very restricted diets.
Vitamin C supports the structural integrity of blood vessel walls. Severe vitamin C deficiency, known as scurvy, weakens the connective tissue that holds blood vessels together, making them fragile enough to leak blood into surrounding tissue. Mild deficiency is more common than outright scurvy and can still contribute to easier bruising, particularly in combination with other risk factors like aging or medication use.
Blood Vessel Inflammation
Vasculitis is a group of conditions in which the immune system attacks blood vessel walls, causing inflammation and thickening. As vessel walls become damaged, blood can leak into surrounding tissue and form hematomas or show up as clusters of red spots on the skin. Vasculitis can affect vessels of any size, from tiny capillaries near the skin’s surface to large arteries deep in the body, and the pattern of bleeding often depends on which vessels are involved.
Because vasculitis is an autoimmune process, it tends to cause symptoms beyond just bruising. Joint pain, fatigue, fever, and skin rashes often accompany the bleeding. The condition can flare and remit, which means hematomas may appear in waves rather than as a constant problem.
Aging and Skin Changes
If you’re over 60 and noticing more bruises on your forearms and hands, the explanation may be structural rather than medical. Senile purpura, sometimes called dermatoporosis, results from decades of sun exposure and natural skin aging. Studies confirm that people with senile purpura have measurably thinner skin than others their age. As the skin and the connective tissue beneath it lose density, the small blood vessels running through them lose their protective cushion. Everyday contact (leaning on a table, carrying groceries) can rupture these vessels and cause flat, purple bruises that look alarming but are generally harmless.
This type of bruising tends to concentrate on sun-exposed areas, especially the backs of the hands and outer forearms. The bruises are often darker than typical bruises and can take longer to fade, sometimes leaving brownish discoloration for weeks.
Supplements You Might Not Suspect
Several popular supplements affect how your blood clots, and many people take them without realizing there’s a connection to bruising. Garlic supplements decrease platelet aggregation in multiple clinical trials. Ginkgo biloba has been linked to cerebral and orbital bleeding in case reports and appears to increase bleeding risk in people already taking blood thinners. Flaxseed oil, grape seed extract, and melatonin have all shown effects on platelet function or clotting activity in randomized trials.
Fish oil is an interesting case. It does have measurable antiplatelet effects, but multiple trials have found these effects don’t consistently translate into increased bleeding for most people. Still, combining fish oil with blood-thinning medications or other supplements on this list could compound the risk. If you’re taking several of these together, the cumulative effect on clotting may be more significant than any single one alone.
How Doctors Investigate
When hematomas keep appearing without clear trauma, the initial workup typically includes a complete blood count with platelet count, a peripheral blood smear (which lets a technician examine your blood cells under a microscope for abnormal shapes or sizes), and two timing tests that measure how quickly your blood clots through different pathways. These tests together can identify whether the problem lies with too few platelets, poorly functioning clotting proteins, or something affecting the blood cells themselves.
Results from this first round of testing usually point toward the next step. Low platelets might prompt a bone marrow evaluation. Abnormal clotting times could lead to testing for specific clotting factor deficiencies. Normal results across the board, combined with the bruising pattern and your age, might point toward a vascular or skin-related cause instead.
Signs That Need Prompt Attention
Most unexplained bruises and small hematomas are not emergencies, but certain features warrant faster evaluation. Hematomas larger than 5 centimeters (roughly 2 inches), those located deep in muscle or in the abdomen, and any hematoma accompanied by pain that worsens rather than improves over days all deserve medical attention. A hematoma that hasn’t started shrinking after two to three weeks is another red flag, as is any combination of unexplained hematomas with bleeding gums, heavy menstrual periods, or blood in the urine or stool. These patterns suggest a systemic problem with clotting rather than a one-off event.