Dilated pupils: Causes and treatments

22 April 2021

Author: Kate Green

dilated pupils

What are dilated pupils?


Dilated pupils, otherwise known as big pupils or large pupils, is the name for when your pupils (the black part in the centre of your eye) grow larger than the average 2-4mm size in bright light. The muscles in your iris (the coloured part of your eye) control the size of the pupil, which is what determines how much light enters your eye. In lighter environments, the pupil contracts to prevent too much light from entering the eye, while in darker environments, the pupil expands to allow as much light as possible into the eye. This happens in order to protect your eyes from overexposure to bright lights as well as to aid your vision in low-light scenarios.


What causes dilated pupils?


As discussed, your pupils primarily dilate in dim lighting to allow you to see well, but your pupils can also dilate for reasons which do not relate to the level of light surrounding you. This can be due to certain types of medicines or because of eye and brain injuries. The dilation of pupils is also called mydriasis.


Medicines which can have this effect are wide-ranging but typically include:

  • Antihistamines
  • Motion sickness tablets
  • Anti-nausea medication
  • Anti-seizure drugs
  • Decongestants
  • Botox


Consumption of illegal recreational drugs can also cause your pupils to dilate. Some of these drugs are:

  • Cocaine
  • LSD
  • MDMA (Ecstasy)


These types of medicines and drugs can cause the muscles in your iris to slow down and not react as well as they should to changes in light. This means that when you move from a dark environment to a light one, your pupils don’t adapt as quickly as they should, if it all, leaving you with dilated pupils. In some cases, withdrawing from taking these drugs after regular use can also dilate your pupils. The function of your iris muscles should then gradually return to normal, in turn improving your dilation function.


Aside from drug or medication use, dilated pupils can also occur as a result of a brain injury, disease or eye injury. Trauma to the head can cause pressure to build in your brain. This can also be caused by a stroke or tumour, affecting the muscles and nerves in your iris. The muscle dysfunction can impact both of your pupils simultaneously, or just cause one pupil to dilate on its own. In the time that your pupils are dilated, you might feel particularly sensitive to light and you would likely benefit from wearing sunglasses in bright areas.


Can mood or attraction dilate your pupils?


Aside from trauma or eye injuries, one of the most common reasons for our pupils dilating is attraction to a person or object. Changes in pupil size are involuntary and eyes are an important element of social interactions with another person. Eye contact provides important social cues, as do subtle eye movements, so it’s no surprise that some studies suggest that pupil size can also change in various social settings.


Typically, it’s thought that when looking at a person you love, your pupils will dilate. Recent research suggests that hormonal changes in women have an effect on pupil dilation too. However, when you pay attention to something or someone, your pupil is also likely to dilate. Since you’re more likely to give more attention to a person you are attracted to or a subject you’re interested in, this could provide another explanation for why your pupils dilate around certain people.


Should you be worried about dilated pupils?


Generally, temporarily dilated pupils are nothing to worry too much about. However, if you’ve taken medication and several hours later your pupils are still dilated, it might be a good idea to get your eyes checked. Similarly, if your dilated pupils stay large when exposed to bright light – rather than just reacting slowly – you should also consider having a medical professional take a look at them.


If one pupil is more dilated than the other, or if you’re dizzy, have a headache, feel confused or have trouble balancing, it could be an indication of something more serious. These could suggest a stroke which needs urgent medical attention, or another type of brain injury or trauma affecting your vision.

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