Brown blood is almost always old blood. When blood sits in your body or on a surface long enough, the iron inside it reacts with oxygen and changes color, the same basic chemistry that turns iron into rust. This process, called oxidation, shifts blood from bright red to dark red, then to brown or even black. In most cases, brown blood is completely normal and not a sign of a medical problem.
Why Blood Turns Brown
Fresh blood is bright red because of hemoglobin, the protein in red blood cells that carries oxygen. Hemoglobin contains iron, and when that iron is bound to oxygen, it gives blood its familiar red color. But iron doesn’t stay in that state forever. When blood is exposed to air or sits in your body without circulating, the iron in hemoglobin spontaneously oxidizes, shifting from its oxygen-carrying form (called ferrous iron) to a form that can no longer carry oxygen effectively (ferric iron). This creates a compound called methemoglobin, and it’s darker in color.
This is essentially the same process as rust forming on metal. The longer blood has been sitting, the more oxidation has occurred, and the browner it looks. It’s why a fresh cut bleeds red but a dried bloodstain on a bandage turns brown within hours.
Brown Period Blood
The most common reason people notice brown blood is during menstruation. Brown period blood is older blood that has spent more time in the uterus before leaving your body. It’s most typical at the very beginning and very end of a period, when blood flow is slower. Because the blood moves through the uterus and vaginal canal at a slower pace, it has more time to oxidize and darken before you see it.
At the start of your period, you might see brown or dark spotting as leftover blood from the previous cycle finally makes its way out. Mid-period, when flow is heavier and blood moves faster, it tends to be bright or dark red. Then as your period tapers off, the flow slows again, and the remaining blood oxidizes to brown or near-black before it exits. All of these colors are normal and simply reflect how long the blood has been sitting in your body.
Brown Discharge After Childbirth
After giving birth, you’ll experience vaginal bleeding called lochia that gradually changes color over several weeks. The first few days involve heavy, bright red bleeding. After roughly a week, the discharge transitions to a pinkish-brown color and becomes less bloody-looking. This pinkish-brown stage, called lochia serosa, typically lasts from about day four through day twelve after delivery. The brown tint follows the same principle: as the uterus heals and bleeding slows, the remaining blood oxidizes before leaving the body.
Brown Blood From a Wound or Bruise
If you’ve noticed brownish discoloration around an old injury, that’s your body breaking down pooled blood beneath the skin. When blood leaks from damaged capillaries after a bruise or cut, it collects in the surrounding tissue. Your body then dismantles the red blood cells and releases the iron they contain. That iron gets stored in a protein called hemosiderin, which has a rusty, brownish-yellow color. Over time, hemosiderin staining can deepen to dark brown or even black.
This is why bruises change color as they heal, cycling from purple to green to yellowish-brown before fading. In most cases, the staining resolves on its own. But in people with chronic venous insufficiency, a circulation problem where damaged valves in the veins prevent blood from flowing efficiently, hemosiderin staining can become persistent. This often appears on the lower legs as brownish patches that don’t go away, because the underlying vein problem keeps causing small amounts of blood to leak from capillaries.
Brown or Cola-Colored Urine
If the brown color you’re seeing is in your urine rather than from a wound or your period, that’s a different situation. Urine can appear brown, tea-colored, or cola-colored for several reasons. Urinary tract infections can cause bleeding that makes urine look red, pink, or brown. Severe dehydration concentrates your urine and can give it a dark amber to brownish appearance. Certain foods like fava beans and some medications can also temporarily darken urine.
More serious causes include liver or kidney problems. When the liver isn’t processing waste efficiently, excess bilirubin (a byproduct of red blood cell breakdown) can make urine dark brown. A condition called rhabdomyolysis, where muscle tissue breaks down rapidly and releases proteins into the bloodstream, can also turn urine brown. If your urine is persistently brown and you can’t attribute it to dehydration or something you ate, it’s worth getting it checked.
When Brown Blood Signals a Problem
In rare cases, the chemistry behind brown blood can become a medical issue on its own. A condition called methemoglobinemia occurs when too much hemoglobin in your blood gets stuck in its oxidized, non-functional form. Under normal conditions, less than 1% of your hemoglobin is in this state. But certain medications, chemical exposures, or genetic conditions can push those levels to 10% or higher. At that point, your blood literally becomes darker and less capable of delivering oxygen to your tissues. The hallmark sign is a bluish tint to the skin, and in severe cases, blood drawn from a vein can appear chocolate brown instead of the usual dark red.
Outside of that uncommon condition, brown blood is rarely cause for concern on its own. The color is almost always a matter of timing: how long the blood has been sitting, how fast it’s moving, and how much oxygen exposure it’s had. If the brown color comes with unusual symptoms like severe pain, foul smell, fever, or persistent bleeding that doesn’t match your normal patterns, those surrounding details matter more than the color itself.