Key Points
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- Multiple types of drugs, including stimulants and hallucinogens, can cause pupil dilation.
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- There are also non-drug-related causes of pupil dilation.
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- Knowing the difference between the two and how to respond is essential for healthier outcomes
Your eyes are an often-overlooked indicator of your overall health status. Acute and chronic illnesses, traumatic injuries, and, in this case, substance use can all cause changes in the eyes that offer important clues.[1]
In the event of dilated pupils, drugs are often the cause (though other reasons can cause dilated pupils as well). Recognizing this subtle physical sign and others can save crucial seconds when trying to help someone who may be using or overdosing on a substance.
Understanding the relationship between drug effects and pupil dilation can help you make crucial judgment calls when fast action is needed. Read on for a comprehensive review of which substances cause dilated pupils, why pupils dilate, and how to respond.
Why Do Pupils Dilate When Drugs Are Consumed?
The iris, or colored part of the eye, is a tiny muscle. Like other muscles, the iris can contract and relax. When the iris relaxes, allowing more light to enter the eye, the pupil dilates.
This process is regulated by the autonomic nervous system, meaning you don’t have to actually “tell” your iris to relax. The sympathetic nervous system (fight or flight) dilates the pupils, while the parasympathetic (rest and digest) nervous system allows the pupils to shrink.[2]
These impressive but often overlooked feats of human function require communication between nerves. Various illicit drugs and other substances can interfere with this communication in ways that cause pupil dilation and other eye changes.
Altering the activity of chemical messengers (neurotransmitters) like dopamine and serotonin is the main mechanism involved.[3]
Not all drugs dilate the pupils in the same way. Powerful stimulants may cause enlarged pupils for hours. Some substances may cause less severe or long-lasting dilation. In most cases, the commonality is some kind of tampering with chemical messengers, whether that is inhibiting or encouraging their activity.
Drugs That May Cause Dilated Pupils
Stimulants
When it comes to dilated pupils, drugs that stimulate the central nervous system activity are the most potent.
Cocaine causes dramatic pupil dilation by blocking the reuptake of chemical messengers like norepinephrine.[4]
Methamphetamine will produce similar side effects by increasing the production of dopamine in the brain.[5]
Amphetamines, including prescription drugs, are also known to cause pupil dilation.[6] Adderall is one example, though pupil dilation often requires misuse in this case.
Ecstasy (MDMA) can cause very pronounced effects on the pupils, including dilation.[7] People who use MDMA often have to wear sunglasses indoors due to the increased light sensitivity from pupil dilation.
Hallucinogens
LSD can cause significant pupil dilation for between 6 and 18 hours or longer.[8] Even smaller amounts can excite the nervous system enough to cause pupil dilation.
Psilocybin mushrooms essentially use the same mechanism to cause pupil dilation. This hallucinogen excites the nervous system and mimics the effects of serotonin.[9]
PCP (phencyclidine) does not commonly cause any significant changes in pupil size, but it’s helpful to add to your general knowledge that this drug can cause erratic eye movements (nystagmus).[10]
Other Substances
Finally, cannabis, synthetic drugs like K2 and spice, and inhalants may all cause mild to severe pupil dilation.[11] Synthetic drugs can be especially unpredictable in their effects on the eyes, often producing a range of symptoms.
Other Causes of Pupil Dilation
Prescription and Over-the-Counter Medications
There are several prescription medications that can cause pupil dilation similar to illicit substances. Antidepressants like SSRIs (selective serotonin reuptake inhibitors) and tricyclics are known to affect pupil size.[12]
Antihistamines like Benadryl can also encourage pupil enlargement.[13] Motion sickness patches may be the most potent when it comes to pupil dilation, often causing the symptom to persist for days due to scopolamine absorption through the skin.[14]
Medical Conditions and Injuries
Traumatic brain injuries and concussions can cause abnormalities of the eye, including pupil function.[15] Migraine and cluster headaches may also cause pupil dilation and other abnormalities.[16]
When a person suffers a stroke or a brain aneurysm, they may experience rapid pupil changes like dilation. Finally, infections attacking the nervous system may cause changes in pupil size.
Environmental and Physiological Factors
With the right environmental trigger, the body will temporarily mirror the effects of a nervous system stimulant. Extreme stress, as in life-threatening situations, will naturally trigger pupil dilation mechanisms. A similar result is observed when someone is extremely excited.
You may be wondering how to differentiate between drug-related and non-drug-related causes of pupil dilation. Thankfully, there are several signs you can look for that will make the distinction more obvious.
How to Recognize Drug Use Through Pupil Changes
Normal vs. Abnormal Pupil Behavior
First, it’s important to understand the main reason behind your eye’s ability to dilate and constrict the pupil.
Healthy pupils should respond consistently to light changes. With more light, the pupil constricts, and with less light, the pupil dilates. This response should only take a few seconds after lighting changes.
In many cases of dilated pupils related to drug use, this response is impaired. The eye often stays dilated no matter how bright the environment you are in. This is a very helpful indicator you can use to assess whether you or someone else is under a drug’s effects.
Another subtle clue offered by the eyes is asymmetry in pupil dilation. In some cases, drug-induced pupil dilation doesn’t affect both eyes to the same extent.
Additional Warning Signs
When assessing someone’s eyes for signs of drug use, pupil size is just one of several points you can check. If you discover pupil dilation, increased sensitivity to light, and/or rapid eye movements, you have a stronger indication that the person is impaired.
Health Implications of Drug-Induced Pupil Dilation
Immediate Physical Risks
Pupil dilation may seem like a benign issue at first, but enlarged pupils reduce the eye’s ability to filter bright light, which can lead to a number of potentially harmful issues.
For example, driving a vehicle is highly discouraged when your vision is impaired by pupil dilation. Your depth perception and ability to react may both be compromised.[17] The eye can also be damaged from prolonged light exposure.
Long-Term Consequences
Consistent substance misuse over months or years can permanently compromise the ability of your pupil to constrict and relax. Some people experience long-term vision problems even after quitting substance use entirely.
Additional consequences of long-term substance use that often accompany eye changes include cardiovascular issues, mental health problems, and worsening addiction.[18]
When Should You Seek Medical Help for Dilated Pupils?
Most people under the influence of a substance will require care for other symptoms (like breathing difficulties or loss of consciousness), not directly related to pupil dilation. However, pupil dilation and other eye changes that last more than a few hours should be evaluated by a healthcare professional as well.
Whether or not pupil dilation is severe, anyone experiencing serious symptoms must seek out medical care immediately.
Find Hope With Professional Treatment
Whether for yourself or someone else, understanding even the subtler signs of substance misuse can save a life. Properly recognized symptoms can open up windows for intervention and healing.
Finding the professional detox and rehab support you need to recover from substance use can have a powerful and transformative effect. Without shame or judgment, a high-quality program will meet your needs with a personalized approach and evidence-based therapies.
Find the support you or your loved one needs and move on with renewed confidence.