Opioid Overdose Damages Oxygen-Sensitive Brain Cells
What oxygen deprivation from near-drowning teaches about overdose and recovery.
Posted February 1, 2026 | Reviewed by Jessica Schrader
When people overdose on opioids, whether they wake up on their own or are revived by Narcan, many doctors assume they are fully recovered. This is a mistake because hypoxia (insufficient oxygen to the brain and body) occurs during overdose, yet often is undiagnosed. Many with opioid use disorder (OUD) experience multiple overdoses. Each overdose adds additional brain cell loss. Research suggests that significantly more evaluation, follow-up, and treatment than currently provided is needed by nearly all overdose patients.
A prepublication of a new study by McKinstry may be definitive enough to change how people with OUDs are treated, especially after overdose. This study showed that people with OUD had significantly smaller hippocampal volumes than healthy controls. In addition, hippocampal volumes were associated with past non-fatal overdoses. Overdose history did not separate OUD subgroups and hippocampal volume by prior overdoses, suggesting overdoses are commonly not recognized. Thus, OUD patients are likely suffering anoxic damage to the hippocampus.
By itself, a history of opioid overdose is associated with lower cognitive function, supporting the idea that major problems may persist even when the acute event appears “reversed.” This new study builds on 50-plus years of published literature on neurocognitive loss caused by damage to the hippocampus after opioid overdose. In 2024, Todaro et al. reported on neurological evidence that non-fatal opioid overdose causes oxygen deprivation injuries rather than mechanical injuries, as from head trauma. Notably, cerebral hypoxia/anoxia damage in OUD and overdose is produced by the same biological cascade seen in near-drowning cases, as well as choking/asphyxiation and cardiac arrest.
Unfortunately, most patient histories do not quantify the number of overdoses a patient has experienced (many patients with OUDs suffer multiple overdoses), the total life amount of naloxone doses/infusions, documented hypoxemia, the number of incidents when patients turned blue, and the use of co-ingestants (benzodiazepines, xylazine, alcohol ) likely because physicians assume the revived OUD patient is “fine."
Near-Drowning vs. Opioid Overdoses
At first glance, near-drowning and opioid overdose appear fundamentally different emergencies. One involves water, aspiration, and suffocation, and the other involves a drug (opioid) suppressing respiration. Yet neurologically, they are both global hypoxic–ischemic brain injuries (HIBI) where the brain is deprived of oxygen and essential nutrients long enough to initiate a cascade of cellular injury. Neurons, especially in high-metabolism regions, are exquisitely sensitive to even tiny changes in oxygen availability and are damaged by oxygen deprivation.
Drowning is much better-studied. Its literature forced clinicians to confront troubling data that early neurologic recovery does not guarantee intact brain function. After a drowning incident, delayed deterioration may occur, even after apparent “complete” recovery or improvement. With near-drowning, long-term cognitive disability is common, even in survivors who seem normal when discharged home.
The brain cannot survive without adequate oxygen, regardless of the cause. The dramatic response to naloxone may create a false sense of reassurance. Clinicians may conclude the event was reversed. However, naloxone is analogous to using the paddles to restore circulation after cardiac arrest: the paddles restarted the heart, but didn’t affect the consequences of a cardiac arrest.
Lessons From Near-Drownings
Drowning medicine has taught us that an early neurologic exam is an unreliable predictor of a patient’s long-term outcome. Patients with initially poor exams can improve; those with good early exams may deteriorate. Accurate prognosis requires time, serial examinations, and attention to evolving patterns.
Memory impairment is a red flag after overdose. New difficulty learning/retaining information after overdose should prompt concern for hypoxic injury. New confusion or behavioral changes weeks after overdose may represent post-hypoxic syndromes rather than intoxication or relapse , although often confused with intoxication/relapse.
Brain neuroplasticity allows partial compensation, but injured circuits—particularly the hippocampus and white matter—may never fully normalize after a near-drowning. Rehabilitation, cognitive therapy , and environmental support may improve functioning, but they cannot erase the injury. The same likely applies to overdose survivors. Lack of systematic follow-up with opioid overdose patients means we may have been underestimating the burden of cognitive disability. Many patients labeled “noncompliant” or “chaotic” may instead have impaired memory and executive function from hypoxic injury, complicating treatment and social reintegration.
One of the most important lessons from hypoxic injury is that certain brain structures are consistently more vulnerable to harm. Survivors of drowning and cardiac arrest often show deficits in memory, attention, and executive function, even when motor and language functions are relatively preserved. The hippocampus, essential for forming new memories, is particularly sensitive to oxygen deprivation. Thus, some patients appear awake, conversational, and oriented, yet cannot retain new information.
Another lesson from drowning and other hypoxic injuries is the phenomenon of delayed post-hypoxic deterioration. Some patients recover consciousness, appear stable, and then decline days to weeks later. As a result, the absence of deficits in the first hours or days after an opioid overdose does not exclude substantial injury.
While drowning and overdose share core biology, overdose introduces additional complexity. Opioid-induced respiratory depression produces not only hypoxemia but also excessive carbon dioxide and respiratory acidosis. Furthermore, many individuals with OUDs experience repeated non-lethal hypoxic episodes—“near-overdoses”—and these incidents may produce chronic deficits in attention, memory, and executive function by cumulatively injuring the brain. From a neurologic perspective, this isn’t one hypoxic insult; it’s a series of hypoxic hits. Neurons surviving an initial insult may become more vulnerable to subsequent insults.
Opioid overdose also differs from drowning because it frequently involves falls. Head trauma during overdose may be unwitnessed and unrecognized, especially if the patient is unresponsive. Even mild traumatic brain injury, when superimposed on hypoxic injury, can worsen outcomes.
OUD alone may be associated with oxygen deprivation-related hippocampal injuries. Neurological injuries common to opioid overdose are similar to a near-drowning. The main take-home is we should not assume administering naloxone to a person who overdosed is sufficient. Instead, the patient should be evaluated over weeks to determine further deterioration in behavior, cognition , or brain function and whether treatment should include neurorehabilitation in addition to medication -assisted treatment and OUD therapies. This perspective reframes overdose from a transient toxicologic crisis into a potential brain injury. That shift has implications not only for diagnosis and prognosis, but also for empathy, as many behavioral and cognitive difficulties seen in overdose survivors may reflect injured brains, not just poor choices.
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Mark S. Gold, M.D., is a pioneering researcher, professor, and chairman of psychiatry at Yale, the University of Florida, and Washington University in St Louis.
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