Why is my pupil large and dilated?

Why do you have an enlarged pupil? Should you be concerned? Well, there are many different reasons for enlarged pupils – the part of the eye that is responsible for letting in light.

An enlarged pupil is known under the medical term as mydriasis. Below we explore some of the reasons for an enlarged and dilated pupil.


Many individuals are surprised to learn that there are a few medications that can dilate the pupils and make them appear very large. This is because the medication can prevent the eye muscles from working correctly so that they don’t get smaller when exposed to bright light. Some drugs include those such as atropine, antihistamines, decongestants, antidepressants and some types of Parkinson’s disease medication.


Illegal drug use can also lead to the symptoms of large and dilated pupils. Some drugs include cocaine, amphetamines, LSD and ecstasy. These drugs change the way in which the eye muscles work. When withdrawing from these types of drugs it is also common to have large pupils until the drug is completely out of the system.

Brain injury

Any type of head injury can cause a build-up of pressure in the brain. The same is true for a stroke or a brain tumour. This build-up of pressure can affect the iris, the coloured part of your eye, which allows the pupil to dilate and close. One or both eyes can be affected and it is common for the eye to have a fixed, dilated pupil.

Cranial nerve palsy

There are several cranial nerves, the second cranial nerve is the optic nerve that is responsible for the brain receiving visual information. The third cranial nerve is responsible for eye movement, including the pupil. Cranial nerve palsy of the third cranial nerve can result in an abnormally large pupil.


Any type of injury to the eye, including blunt trauma, can cause damage to the nerves that control the iris and pupil. This type of trauma may result in a delayed response to the eye being exposed to bright light, resulting in large and dilated pupils for an exposed period of time.

Make an appointment with your GP

If you are aware that your pupils are constantly dilated, then make an appointment with your doctor or optometrist. It is really important to make an appointment with an eye specialist if one of your eyes has a larger and dilated pupil. They will be able to determine the underlying cause and diagnose any health conditions.


Always consult a qualified optometrist to confirm a diagnosis!

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