Why and how does eye colour change?

Eye colour is a fascinating part of human genetics and biology. Most people are born with a particular eye colour, but it is not unusual for it to change over time. This guide explains why eye colour changes and the factors that influence it.

1. Genetics

Genes play the main role in deciding eye colour. The amount and spread of melanin, the pigment that gives the eye its colour, are set by genes inherited from both parents. Eye colour is usually described as shades of brown, blue, green or hazel, with variations depending on the combination of genes you inherit.

2. Melanin production

The amount of melanin in the iris, the coloured part of the eye, decides its shade. More melanin gives darker eyes such as brown; less melanin gives lighter colours such as blue or green. Changes in melanin production over time, influenced by age, hormonal changes and sunlight, can shift eye colour.

3. Age

Eye colour often changes with age, particularly in infancy and early childhood. Babies are frequently born with blue or grey eyes because their irises contain little melanin. As melanin production increases during the first few years of life, the eyes may darken or turn brown, green or hazel.

Large changes in eye colour after childhood are rare, but they can happen because of the other factors below.

4. Sunlight exposure

Sunlight can affect melanin production in the iris and change eye colour over time. Long exposure to ultraviolet (UV) rays can stimulate melanin production, so the eyes may darken or develop flecks of colour. Limited sunlight exposure, on the other hand, may go with lighter or faded eye colours.

5. Hormonal changes

Hormonal changes, particularly during puberty, pregnancy and menopause, can influence melanin production and cause temporary changes in eye colour. Higher levels of oestrogen and other hormones can stimulate melanin and darken the iris. Hormonal changes linked to ageing or to certain medical conditions may also affect eye colour.

6. Eye diseases and conditions

Some eye diseases, such as glaucoma, Horner's syndrome and pigment dispersion syndrome, can change eye colour by damaging the iris or altering how melanin is distributed. Treatments for these conditions, including medicines or surgery, may also affect eye colour, temporarily or permanently. Read more about glaucoma treatment.

7. Medicines

Some medicines can affect melanin production or the structure of the iris and change eye colour as a side effect. For example, certain prostaglandin analogue eye drops used to treat glaucoma commonly darken the iris.

8. Trauma or injury

An injury to the eye, such as a blow to the head or a penetrating injury, can damage the iris and change its appearance and colour. Although it is rare, significant trauma or eye surgery can change eye colour permanently through scarring or tissue damage.

When should you see an eye doctor?

A change in eye colour in early childhood is usually a normal part of development. Because significant changes after childhood are rare and can be linked to eye disease, medicines or injury, a noticeable change in an adult is worth having checked by an ophthalmologist (eye specialist).

In short

Eye colour is decided by complex genetic factors and influenced by environmental, hormonal and physical ones. Most people keep the eye colour they are born with, but it can change over time with age, sunlight, hormones and some eye diseases or treatments. Understanding these reasons helps explain the variety and complexity of human biology.