What Causes Unexplained Bruising?

The short answer: most unexplained bruises come from one of three ordinary things: thin, sun-damaged skin in older adults, a medication or supplement that slows clotting, or a bump you did not notice. A smaller group has a real bleeding problem: too few platelets, a missing clotting protein such as von Willebrand factor, liver disease, or a vitamin deficiency. Rarely, new bruising is the first sign of leukemia. The bruises themselves rarely tell you which it is. The company they keep does: petechiae, nosebleeds, bleeding gums, fatigue, fever, or a drug started in the last few weeks.

What counts as normal bruising?

Normal bruises sit on the shins and forearms, follow a knock you can name or plausibly forgot, and fade through red, blue, green, and yellow over about two weeks (MedlinePlus). A 2016 review in American Family Physician notes that 26% to 45% of healthy people report easy bruising, nosebleeds, or gum bleeding when asked (Neutze & Roque, AFP 2016).

Bruising moves from normal to worth checking when:

• It appears on the trunk, back, face, or upper arms, where accidental knocks are unusual.
• Bruises are large after trivial or no contact, or many appear at once.
• A lump forms under the bruise, or it hurts out of proportion to its size.
• It comes with other bleeding: long nosebleeds, gums that bleed when brushing, heavy periods, or bleeding that will not stop after a cut or dental work.

Why do older adults bruise so easily?

The most common cause of "bruises I cannot explain" after 50 is the skin, not the blood. Years of sun exposure thin the dermis and weaken the tissue around small vessels, so light pressure on the forearm leaves a flat purple patch called actinic or senile purpura. A StatPearls review estimates it affects about 12% of people over 50 and 30% of people 75 or older, most often on the backs of the hands and outer forearms (StatPearls: Actinic Purpura). These patches clear in one to three weeks and carry minimal bleeding risk. Steroid creams and oral corticosteroids thin the skin further and are worth reviewing if the purpura is getting worse.

Which medications and supplements cause bruising?

This is the highest-yield question in the workup. The AFP review groups the culprits by mechanism:

• Anticoagulants: warfarin, heparin, enoxaparin, dabigatran, rivaroxaban, apixaban.
• Drugs that blunt platelets: aspirin, NSAIDs such as ibuprofen and naproxen, clopidogrel, SSRIs, fish oil.
• Corticosteroids, which break down collagen and thin the skin.
• Drugs that lower the platelet count: alcohol, some antibiotics, carbamazepine, quinine, thiazide diuretics, valproic acid.

Two of these surprise people. SSRIs deplete serotonin inside platelets and make them less sticky. A meta-analysis of 42 observational studies found 41% higher odds of bleeding in SSRI users (odds ratio 1.41, 95% CI 1.27 to 1.57) (Laporte et al., 2017). The absolute risk is small and rises most when an SSRI is paired with an NSAID or anticoagulant.

Fish oil is the other. It appears on most bleeding-risk lists, but a 2024 meta-analysis of 11 randomized trials with 120,643 patients found no increase in bleeding events with omega-3 supplements (rate ratio 1.09, 95% CI 0.91 to 1.31). Only high-dose purified EPA showed a modest rise, about 0.6% in absolute terms (JAHA 2024). Ginkgo is less studied and may raise bleeding risk in people on warfarin (NCCIH). Our sister site supplements.md covers the evidence on common supplements.

The practical rule: if bruising started within weeks of a new drug, dose increase, or supplement, that is the first suspect. Do not stop a prescribed blood thinner on your own. The reason you take it is usually more dangerous than the bruise.

Could it be low platelets?

Platelets plug small vessel leaks before they become bruises. The normal count is 150,000 per microliter or higher. StatPearls sets the thresholds: 100,000 to 150,000 is usually silent; 50,000 to 100,000 can cause easy bruising; below 50,000 raises bleeding risk with surgery; below 10,000 risks spontaneous hemorrhage (StatPearls: Thrombocytopenia).

Causes include medications, viral infections such as HIV and hepatitis C, an enlarged spleen from liver disease, and blood cancers. In immune thrombocytopenia (ITP), the immune system destroys platelets. In children it usually follows a virus and 85% recover within three months. In adults it tends to be chronic and affects women three times as often as men (StatPearls: ITP).

The clue that points to platelets rather than skin is petechiae: pinpoint red or purple dots, often on the lower legs, that do not blanch when pressed. The AFP review lists petechiae, nosebleeds, and gum bleeding as the classic platelet pattern.

Could it be a clotting factor problem?

The most common inherited bleeding disorder is von Willebrand disease. It affects about 1% of the population by lab testing, though clinically significant disease is closer to 125 per million, and it affects men and women equally (StatPearls: Von Willebrand Disease). The pattern is mucosal: recurrent large bruises, nosebleeds, prolonged bleeding after dental work, and heavy periods. The AFP review notes that 29% of women who seek care for heavy menstrual bleeding have an underlying bleeding disorder.

Hemophilia is rarer. Hemophilia A occurs in about 1 in 5,000 male births and hemophilia B in about 1 in 30,000; roughly 30% of cases arise from a new mutation with no family history (StatPearls: Hemophilia). Its signature is deep bleeding into joints and muscles, usually found in early childhood.

What about the liver and vitamins?

The liver makes most clotting proteins, and cirrhosis also traps platelets in an enlarged spleen. The NIDDK lists "bruising and bleeding easily" among the symptoms of failing liver function, alongside jaundice, swollen legs, and fluid in the abdomen (NIDDK).

Vitamin K activates clotting factors II, VII, IX, and X. Adults become deficient through fat malabsorption (celiac disease, pancreatic insufficiency, bariatric surgery), prolonged antibiotics, warfarin, or a very restricted diet. The result is bruising, petechiae, nosebleeds, and a prolonged prothrombin time (StatPearls: Vitamin K Deficiency).

Vitamin C deficiency is rarer but real. About 7.1% of Americans are deficient, and scurvy symptoms appear within 4 to 12 weeks of inadequate intake: bruising, bleeding gums, bleeding around hair follicles on the legs, and corkscrew-shaped hairs. Smokers, heavy drinkers, and people on very limited diets are at risk (StatPearls: Vitamin C Deficiency).

Cushing syndrome, from excess cortisol, causes paper-thin skin, easy bruising, and wide purple abdominal stretch marks. Long-term corticosteroid treatment is the usual cause (StatPearls: Cushing Syndrome).

Red flags: when does bruising need prompt evaluation?

Book an appointment this week if unexplained bruising comes with any of the following:

• Bruises without trauma in unusual places: trunk, back, face, or upper arms.
• Petechiae that do not fade under pressure.
• Bleeding elsewhere: frequent nosebleeds, bleeding gums, blood in urine or stool, or periods that soak through protection hourly.
• Fatigue, fever, night sweats, or unintended weight loss. The NCI lists tiredness, fever, pallor, and easy bleeding as early signs of acute leukemia, often developing over the 4 to 6 weeks before diagnosis (NCI).
• Swollen lymph nodes or a swollen abdomen.
• Bruising that began within weeks of a new medication or a dose change.
• Jaundice, swollen legs, or heavy alcohol use, which point toward the liver.

Go to an emergency department for a bruise with severe pressure or pain in a limb, bleeding you cannot stop, or a fall or head injury while on a blood thinner. Our red flags guide covers warning signs that should never wait, and our post on when to see a doctor sets thresholds for less urgent complaints.

What tests will a doctor order?

The AFP review recommends a short first panel:

• Complete blood count with a peripheral blood smear, to count platelets and look at the cells.
• Prothrombin time (PT/INR), which tests the vitamin K-dependent factors and liver function.
• Activated partial thromboplastin time (aPTT), prolonged in hemophilia and often in von Willebrand disease.
• Liver and kidney function tests.

If those are normal but the bleeding history is convincing, the next step is a von Willebrand panel (vWF antigen, ristocetin cofactor activity, and factor VIII activity) and a hematology referral. Bring a list of every drug and supplement you take, and note any heavy bleeding after surgery, dental work, or childbirth. If tiredness is part of the picture, our fatigue guide covers how anemia shows up on the same blood count.

The bottom line

Most unexplained bruising is sun-thinned skin, a blood-thinning drug or supplement, or a knock you forgot. Start with the medication list and the location of the bruises. Forearms and shins in someone over 50 are almost always benign. Trunk and face bruises, petechiae, nose or gum bleeding, heavy periods, or fatigue and fever are the combinations that need a blood count and clotting tests soon. One visit and four inexpensive tests separate the harmless causes from the ones that matter.

Last updated: August 2026. This article is for informational purposes only and does not constitute medical advice. It cannot diagnose your symptoms. Do not stop a prescribed blood thinner without speaking to the prescriber, and seek prompt care for bruising with petechiae, uncontrolled bleeding, fever, or severe pain.