What Drugs Cause Dilated Pupils?

There are many signs we can look for that indicate that someone has been using drugs. In law enforcement and in medical communities that deal with substance misuse, the dilation level of the eyes is considered to be a key marker of illicit drug use, and can even shed light on the particular drug that was used.

What Drugs Cause Dilated Pupils?

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, which controls 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 in the dilator.

Certain drugs, most commonly psychotropic stimulants, have a large effect on both and cause the pupils to respond by dilating. Drugs can affect the parasympathetic or sympathetic nervous systems individually or in combination, depending 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, causing the pupils to dilate.

There are many drugs that can work on the brain’s neurotransmitters and affect the dilation of the 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 dilation. Adrenergic receptors, another neurotransmitter, are affected by other drugs (such as marijuana) when dopamine is released and, once more, results in dilated pupils.

Simply put, drugs affect muscles in the eye that control the amount of light which is 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 of this variance, analyzing pupil dilation alone is an imperfect way to assess sobriety. As a result, officials have learned to look for other indicators, such as heavy sweating, dry mouth, excessive activity and the like.

In the event there is a suspicion of substance misuse, there is a tool to help make a determination. An official chart used mostly by law enforcement and emergency medical teams, called the “Drug Recognition Card,” shows images of pupils reacting to various drugs. 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.

The chart lists the drug categories in columns and the physical conditions noticed, such as gaze, convergence, state of pupil dilation or light reaction, in order to determine a likely match. It also features a scale of pupil dilation that, when held up to the person, 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; however, they are not always as reliable as the one provided by the IACP.