Heterochromia, having two different eye colors or more than one color within a single eye, affects roughly 1% of people worldwide. Most cases are genetic and present from birth, caused by harmless mutations in the genes that control melanin production in the iris. But heterochromia can also develop later in life from eye injuries, certain diseases, or even medication side effects.
How Iris Color Works
Your eye color depends on how much melanin, the same pigment that colors your skin and hair, is present in your iris. Brown eyes have a lot of melanin. Blue eyes have very little. Green and hazel fall somewhere in between. Heterochromia happens when melanin is distributed unevenly, either between your two eyes or within different zones of one eye.
The Three Types of Heterochromia
Complete heterochromia is the most recognizable form: each eye is a distinctly different color, like one brown eye and one blue eye. Sectoral heterochromia means a segment of one iris is a different color from the rest of that same eye, creating a wedge or patch of contrasting color. Central heterochromia shows up as a ring of one color around the pupil with a different color radiating outward, giving the iris a two-toned, starburst appearance. Central heterochromia is the most common of the three.
Genetic Causes Present From Birth
The most common reason someone has heterochromia is a random, harmless genetic mutation that affects how melanin is made, transported, or stored in the iris. These mutations happen spontaneously during development, meaning no one else in the family needs to have it. In other cases, heterochromia runs in families and is passed down as an autosomal dominant trait, which means inheriting just one copy of the gene variant from either parent is enough.
Because infant eye color can continue changing for up to three years after birth, congenital heterochromia may not always be obvious right away. Babies are often born with lighter eyes that gradually darken as melanin accumulates in the iris. If one eye darkens at a different rate or to a different degree than the other, the mismatch becomes apparent over those first months or years.
Medical Conditions Linked to Heterochromia
In a small number of cases, heterochromia at birth signals an underlying condition rather than a standalone trait. Several syndromes include heterochromia among their features.
Waardenburg syndrome is one of the better-known examples. It’s a group of genetic conditions that affect pigmentation of the hair, skin, and eyes, along with hearing. People with Waardenburg syndrome often have very pale blue eyes, two different-colored eyes, or segments of different colors within one eye. Other signs include a distinctive patch of white hair, premature graying, and moderate to profound hearing loss in one or both ears. The syndrome has four types. Types I and II look similar, though type I typically involves widely spaced eyes. Type III also causes abnormalities of the arms and hands. Type IV combines Waardenburg features with Hirschsprung disease, an intestinal disorder that blocks normal bowel function.
Congenital Horner syndrome is another cause. It results from nerve damage affecting one side of the face and can prevent the affected eye from developing full pigmentation, leaving it lighter than the other. Other congenital conditions associated with heterochromia include Parry-Romberg syndrome, Sturge-Weber syndrome, and neurofibromatosis type 1.
How You Can Develop It Later in Life
Acquired heterochromia, the kind that shows up after birth, has a different set of causes. These fall into three broad categories: inflammation, injury, and medication.
Chronic Eye Inflammation
Fuchs heterochromic iridocyclitis is a low-grade, long-lasting inflammation inside the eye that gradually damages the iris. Over time, the iris tissue develops a moth-eaten appearance as its surface structure breaks down and pigment cells are lost. In people with darker eyes, this makes the affected iris noticeably lighter. Interestingly, in people with light-colored eyes, the process can work in reverse: the pale outer layer of the iris wears away and exposes the darker pigment layer underneath, making the affected eye appear darker than its partner. The iris damage typically precedes the visible color change, so the heterochromia develops gradually over months or years.
Eye Injury
Trauma to the eye can disrupt pigment cells in the iris. A direct blow, a penetrating injury, or even surgery can cause melanin to scatter, clump, or be lost entirely. Bleeding inside the eye can deposit iron into iris tissue, staining it a darker shade. These changes are usually limited to the injured eye, creating an obvious mismatch with the unaffected side.
Glaucoma Medication
A class of eye drops used to treat glaucoma, called prostaglandin analogs, can gradually darken the iris of the treated eye. This is a well-documented class effect across these medications, not limited to any single brand. The drops work by stimulating melanin production in the iris, and when only one eye is being treated, the result can be a noticeable color difference between the two eyes. The darkening tends to be most visible in people with hazel or mixed-color irises and is generally considered irreversible even after stopping the drops.
When Heterochromia Needs Attention
If you’ve had two different-colored eyes your whole life and no other symptoms, the cause is almost certainly a benign genetic variation. No treatment is needed, and it won’t affect your vision.
The situation is different when heterochromia appears for the first time in adulthood, or when a baby’s mismatched eye colors come with other signs like hearing difficulty, a drooping eyelid, or a difference in pupil size. New-onset color changes in one eye can point to inflammation, pigment loss, or other conditions that benefit from early evaluation. The color change itself isn’t harmful, but the underlying cause may need to be addressed to protect your vision.