Dilated pupils are when the black part in the centre of your eye (the pupil) increases in size. This normally happens when light levels drop, but it can also be a sign of other eye conditions if it happens when you’re in bright light and they don’t shrink back down to their normal size.

What causes pupils to dilate?

The muscles in your iris (the coloured part of your eye) control your pupil size. Your pupils change in size depending on how bright the light is around you. In low light, your pupils open up (dilate) to let in more light, but when it’s bright, they get smaller (constrict) to let in less light. A healthy pupil size will normally range from 2-4mm in normal light up to 8mm in low light.

If your pupils dilate without any change in light it is called ‘mydriasis’ and can be caused by a variety of things, including:


A few medicines can affect the iris muscles and prevent them from constricting your pupils when light hits them. These include some antidepressants, antihistamines, decongestants and even motion sickness tablets. Some illegal drugs can also have the same effect. If you notice your eyes are staying dilated as a result of medication, you should go back to see your GP.


Some infections of the eye and nervous system can affect the ability to control the pupils, an example would be the virus that causes chickenpox or shingles. In some cases, infections can happen after eye surgery that affect the iris’s ability to control the pupils, for example, after cataract surgery or corneal transplant.

Eye injury

An eye injury can damage nerves or the muscles in your iris that control your pupil size. If you injure your eye it’s important to seek medical attention.

Brain injury or disease

If you have an injury or disease that affects your brain (for example head trauma, stroke or tumour) the pressure can build in your head and affect the ability of the muscles in your iris to control your pupil. This is why you often see doctors shining lights in a patient’s eyes to see if the pupils constrict (as they should) or stay dilated.


Often used in advertising to demonstrate beauty, dilated pupils are a sign that we are attracted to someone. This is because when we see someone attractive our brains release dopamine which can cause our pupils to dilate.

Dilated pupils and other eye conditions

Dilated pupils can also be an indication of other, more serious eye conditions and infections, including:

Relative afferent pupillary defect (RAPD) – in this case, the pupils actually dilate in response to light instead of shrinking – in one or both eyes. This is a sign that a disease is affecting the retina or optic nerve.

Glaucoma – affects the optic nerve and causes pressure to build up in your eye. If the pressure rises significantly it can affect the iris’s ability to control the pupil. 

Uvetis or iritis– there are different types of inflammation which can affect the eye depending on the area of the uvea (the middle layer of the eye which includes the iris) that is inflamed, and how long it lasts for. It can be painful and make you sensitive to light because the pupilsstay dilated. 

In each of these cases, fixed dilated pupils are an indication that something is not quite right with your eyes and needs looking into by an optician – the earlier the better too. Book an appointment here.

How do opticians test your pupillary responses?

To check your pupils, the optician will run a few tests including shining a light into your eyes to see if they constrict and then dilate when the light moves away. They’ll also watch how your pupils react as you focus on objects near and far.

How can you treat dilated pupils?

The treatment for fixed dilated pupils (mydriasis) depends on the underlying condition that causes them. In the meantime, your optician or GP may recommend opaque contact lenses or light-sensitive sunglasses to help relieve the symptoms. Or, if medication is the cause, your GP may prescribe you an alternative. In some cases where the pupil dilation is caused by injuries to the brain and eyes, you may need surgery to repair any damage to the nerves or eye structures. So it’s important to have your symptoms checked as soon as possible.

When to see your optician about dilated pupils

If you notice that your pupils are fixed and dilated, or you’ve experienced a recent change in your vision, it’s important to book an appointment with your optician or GP as soon as possible to investigate further.

For more information on a range of other common eye conditions, visit our eye conditions page.