Heroin is a powerful opioid derived from the opium poppy. It is rapidly converted into morphine, which then binds to opioid receptors in the brain. This interaction slows brain activity, producing feelings of intense pleasure, relaxation, and drowsiness. The body exhibits several physical changes following ingestion, and the eyes are one of the most reliable indicators of recent opioid use.
The Defining Ocular Sign
The most recognizable sign of heroin ingestion is the constriction of the pupils, known medically as miosis. This reaction causes the pupils to shrink to a small, “pinpoint” size. Pinpoint pupils are a hallmark of opioid use and typically affect both eyes equally (bilateral).
This constriction is maintained even in dim lighting, where pupils normally dilate, and they may be less responsive to light changes. Following heroin use, the pupils can become as small as two millimeters or less.
Other changes can accompany miosis. The eyelids may appear heavy or droopy (ptosis), contributing to a drowsy appearance. The eyes may also appear bloodshot or red due to the drug’s impact on the circulatory system.
Biological Mechanism Behind Pupil Changes
Miosis results from how heroin interacts with the nervous system’s control over pupil size. Heroin and morphine are strong agonists for mu-opioid receptors throughout the central nervous system. Activating these receptors stimulates the parasympathetic nervous system, which governs “rest and digest” functions.
In the brainstem, this stimulation affects the Edinger-Westphal nucleus. Opioid binding increases the activity of this nucleus, which controls the parasympathetic output to the eye. This increased signaling causes the iris sphincter muscle to contract forcefully.
The sustained contraction of the iris muscle constricts the pupil. This mechanism makes miosis a reliable physical marker of opioid presence, and the effect is dose-dependent.
Additional Physical Indicators
Heroin intoxication presents with several other physical indicators. A common sign is “nodding,” where the person alternates between being awake and suddenly falling into deep drowsiness. This cycle of repeated drooping and jerking awake occurs rapidly.
As a central nervous system depressant, heroin causes slow and shallow breathing, known as respiratory depression. This is the primary cause of death in overdose situations. The skin often appears flushed and warm, and many users experience intense itching.
Other signs include dry mouth and slurred speech due to the sedative effects. The body may also display a general lack of coordination and heaviness in the limbs.
Duration and Differential Considerations
The duration of heroin-induced miosis is related to the drug’s half-life and the amount consumed, often lasting several hours. The effects of a single dose typically last between three and five hours, and pupillary constriction usually persists throughout this period. The exact timeline is variable based on the drug’s potency and the individual’s tolerance level.
Pinpoint pupils alone are not definitive proof of heroin use, as other factors can cause miosis. These differential causes include:
- Certain prescription medications, such as high blood pressure drugs (e.g., clonidine).
- Specific eye drops used to treat glaucoma.
- Exposure to toxins like certain pesticides or nerve agents.
- Specific medical conditions, including a pontine hemorrhage or Horner’s syndrome.
In cases of severe heroin overdose leading to hypoxia, the pupils may paradoxically dilate. This is a critical sign of a life-threatening emergency.