What Drugs Cause Dilated Pupils?

There’s a whole lot of truth in the adage “the eyes don’t lie.” So much so that, in law enforcement and medical communities that deal with substance misuse, the dilation level of an individual’s eyes is considered a key marker indicating that there has been consumption of an illicit drug and can often lend assistance in identification of the drug used.

Dilation of the pupil (mydriasis), or opening of the iris, is caused by the activation of two muscle groups in the eye: the iris sphincter and the iris dilator. The body’s parasympathetic nervous system (providing control of a person’s autonomic bodily processes when at rest) is what triggers the sphincter response. The sympathetic nervous system, which controls the body’s fight-or-flight response, triggers action of the dilator.

Certain drugs, most commonly psychotropic stimulants, have a large effect on both systems causing the pupils to respond by dilating. Drugs can affect the parasympathetic or sympathetic nervous systems individually or in combination and depends on the type of drug taken. This occurs when elements of the drugs affect neurotransmitters in the brain that work in part to control mydriasis – thus allowing the pupils of the user to dilate to differing degrees.

Drugs that Cause Dilated Pupils

There are many drugs that can work on the brain’s neurotransmitters and affect the dilation of pupils. These include SSRI antidepressants, amphetamines, MDMA, psilocybin, LSD, ecstasy, cocaine, and mescaline. In the case of these drugs, serotonin (a brain chemical affecting mood) agonizes the 5-HT2A receptors in the brain and kicks off the dilation. Adrenergic receptors, another neurotransmitter, is affected by other drugs (such as marijuana) when dopamine is released, again affecting the eyes so the pupils become dilated.

Simply put, drugs take affect on the muscles in the eye that control the amount of light being allowed in. Since many drugs affect perception in the brain, the reaction to light can be altered, allowing the pupils to react in atypical fashion as to what is expected. Because this alone is an imperfect way to check against sobriety, officials have also learned to take other factors into play, such as heavy sweating, dry mouth, excessive activity, and the like.

Dilated Pupils on Drugs – Seeing the Difference

In the event there is suspicion of substance misuse, there is a tool to help at least part of the determination. An official chart showing pupils on drugs is the “Drug Recognition Card,” used mostly by law enforcement and emergency medical teams. This card is based on standards set by the International Association of Chiefs of Police (IACP) and provides a means to readily assess likely substance use categories (depressants, stimulants, hallucinogens, phencyclidine, narcotics, inhalants, cannabis) based on pupil diameter.
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The chart lists the drug categories in columns, and the physical conditions noticed (horizontal/vertical gaze, convergence, state of pupil dilation, light reaction rate, etc.) so a likely match can be obtained. It also features a scale of pupil dilation that, when held up to the user, provides a physical, visual gauge for referencing how much or little dilation there is. Other charts showing the effects of drugs on pupil dilation do exist though, while helpful, they are not always as reliable as the one provided by the IACP which can be readily purchased online for home use.

Written for Advanced Recovery Systems