Over time, the sun’s harmful UV rays can cause collagen—the spongy tissue in the second layer of your skin—to dissolve. Collagen acts like a cushion to protect the network of blood vessels underneath, explains Cynthia Bailey, M.D., diplomat of the American Board of Dermatology.

Without enough of that padding, your blood vessels are more prone to rupture, which causes the black and blue marks.

And it can happen quicker than you might think. Theoretically, one intense summer could be enough to have an effect, Dr. Bailey says. So while you can’t take back your college spring breaks on the beach, you can make sure you’re not adding to the damage. Always wear sunscreen—when you’re outside, and reapply regularly.

Even if you don’t spend hours a day in the sun, over time, unprotected exposure to the sun’s UV rays will still cause your collagen to break down and get thinner—making you more susceptible to bruising.

Age-related collagen breakdown happens at a different rate for everyone, and it depends on the thickness of your skin and how susceptible you are to sun damage. For instance, fair-skinned people are usually more vulnerable than darker-skinned people.

“Some people won’t experience [age-related bruising] until they’re very old, but others could experience it in their 40s or 50s,” says Dr. Bailey.

There’s no foolproof way to prevent age-related skin thinning and the bruising that might come with it. But using sunscreen can preserve your skin’s collagen for longer, Dr. Bailey says.

Booze doesn’t just make you clumsier. After just a few drinks, your skin becomes more susceptible to bruising after a trip, fall, or bump.

Alcohol is a vasodilator, meaning it causes your blood vessels to temporarily relax and expand. This widening increases blood flow through your body, including near the surface of your skin, says Scott Swartzwelder, Ph.D., coauthor of Buzzed: The Straight Facts About the Most Used and Abused Drugs from Alcohol to Ecstasy.

More blood flowing through your vessels means that more blood can escape if a blood vessel happens to get ruptured—say, if your leg bumps into the corner of a table, Swartzwelder says. Couple that with the fact that you probably are a little less agile on your feet after a couple pints, and it’s no wonder you wake up a black and blue mess the night after a pub crawl.

For the average healthy guy drinking one to two drinks per night, a booze-related bruise isn’t cause for concern. It should go away on its own in 3 to 4 days.

Blood-thinning medications like warfarin and heparin—which are used for conditions like abnormal heart rhythm and congenital heart defects—can all put you at greater risk for bruising.

Anticoagulants increase the time for blood to clot—or to turn from a liquid to a gel-like substance that stops bleeding. That’s why patients on blood thinners tend to bruise easier and more severely, explains says Dr. Movassaghi.

OTC pain relievers like aspirin, ibuprofen, and naproxen can also have a blood-thinning effect. “They inhibit a specific enzyme required for blood clotting,” says Dr. Movassaghi.

Avoid using them for more than a week at a time, unless under the direction of your doctor.

Vitamins C, K, and B12 all play a role in helping blood clot. When you don’t get enough, even a light bump could result in a big bruise, says Alissa Rumsey, M.S., R.D., founder of Alissa Rumsey Nutrition and Wellness.

Nutritional deficiencies aren’t common in young, healthy guys who eat a varied diet, Rumsey says. But if you live on junk food or don’t eat a lot of fruits and vegetables, you could be at risk for missing vitamins C or K. Other signs of vitamin C deficiency include fatigue and slow wound healing; signs of a vitamin K deficiency include nosebleeds or bleeding gums.

You might also have a harder time getting enough B12 if you’re vegan or vegetarian, since it’s mainly found in animal proteins. Other signs of B12 deficiency include numbness or tingling in the hands or feet, weakness, and fatigue. If you think you have a gap in your diet, consider talking with your doctor or a registered dietician.

Usually, a few unexplained bruises are nothing to worry about. But in rare cases, they could be a sign of a serious illness, like type 2 diabetes or leukemia.

If you’ve had unusual or unexplained bruising for more than two weeks, talk to your doctor to rule out anything major, says Dr. Movassaghi.

High blood sugar causes the skin to heal less effectively, upping the risk for severe bruises or bruises that take longer to heal, Dr. Movassaghi says. And people with leukemia have fewer platelets in their blood to plug bleeding blood vessels, which can also increase bruising susceptibility.

So if you’ve been experiencing more unexplained bruising than usual, your doctor should check it out. He or she will perform blood tests or other diagnostic tests to determine the cause.

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