Acetaminophen (known as paracetamol outside the United States) is not recognized as a hallucinogen at standard doses, and hallucinations are not listed among its established side effects. That said, post-marketing safety databases have flagged rare signals linking acetaminophen to abnormal neuropsychiatric behavior, and the drug turns out to have more going on inside the brain than most people realize. The story gets more complicated when you factor in overdose, combination products that bundle acetaminophen with other active ingredients, and the particular vulnerability of older adults.
What Safety Databases Actually Show
Pharmacovigilance systems collect reports of unusual reactions to drugs after they reach the market. These databases are enormous and catch patterns that clinical trials, which typically involve hundreds or thousands of participants, might miss. A study that mined one such database for neuropsychiatric adverse events found that signals for “abnormal behaviour” were detected for acetaminophen, alongside drugs like oseltamivir (Tamiflu) and montelukast (Singulair), both of which already carry warnings about neuropsychiatric effects.1PubMed Central. Drug-induced Neuropsychiatric Adverse Events Using Post-Marketing Surveillance A “signal” in this context does not prove acetaminophen caused hallucinations or other psychiatric symptoms in those patients. It means the association came up often enough to warrant attention. These reports are voluntary, uncontrolled, and can be confounded by the illness being treated, other medications taken at the same time, or the patient’s underlying health. Still, the signal exists, and it is worth understanding what might explain it.
Acetaminophen Does More in the Brain Than You’d Expect
Most people think of acetaminophen as a simple pain reliever that works “somewhere in the body” without much specificity. In reality, much of its pain-relieving action happens inside the central nervous system, and the pathways it touches overlap with systems involved in mood, perception, and consciousness.
Once acetaminophen enters the brain, it gets converted into a metabolite called AM404. This compound activates a receptor called TRPV1 and triggers a signaling chain that ultimately involves cannabinoid receptors, the same receptor family that THC targets.2PubMed Central. An Updated Review on the Metabolite (AM404)-Mediated Central Mechanism of Action of Paracetamol (Acetaminophen): Experimental Evidence and Potential Clinical Impact A 2025 paper added another layer to this picture by showing that acetaminophen directly inhibits an enzyme called DAGLα, which reduces production of a key endocannabinoid molecule.3Cell Reports Medicine. Acetaminophen inhibits diacylglycerol lipase α, reducing endocannabinoid signaling and altering nociception In plain terms, acetaminophen is tweaking your brain’s own cannabis-like signaling system every time you take it. At normal doses, these effects are subtle and contribute to pain relief. Whether they could, in unusual circumstances, produce perceptual disturbances is not established, but the biological machinery to affect perception is clearly being engaged.
Acetaminophen also raises serotonin levels in the brain. Animal studies found significant increases in serotonin across multiple brain regions after acetaminophen administration.4PubMed. Effects of acetaminophen on monoaminergic systems in the rat central nervous system This serotonergic involvement isn’t just an animal curiosity. In a human study, when researchers gave subjects serotonin-blocking drugs alongside acetaminophen, the pain relief from acetaminophen was completely abolished, confirming that serotonin is a key part of how the drug works in people.5PubMed. Analgesic effect of acetaminophen in humans: first evidence of a central serotonergic mechanism The broader picture emerging from research is that acetaminophen’s pain relief involves cannabinoid, serotonergic, opioidergic, and other neural pathways simultaneously.6PubMed Central. The Contribution of Serotonergic Receptors and Nitric Oxide Systems in the Analgesic Effect of Acetaminophen: An Overview of the Last Decade
None of this means a standard dose of acetaminophen will make you see things. But it does undercut the assumption that acetaminophen is neurologically inert. Any drug that touches serotonin, cannabinoid, and opioid-related pathways has at least a theoretical potential for neuropsychiatric effects in the right (or wrong) circumstances.
Combination Products Are Often the Real Culprit
When someone reports hallucinations after taking “Tylenol” or a cold medicine that contains acetaminophen, the acetaminophen itself may not be the problem. Many over-the-counter products combine acetaminophen with other active ingredients, and those other ingredients have well-documented potential for hallucinations and other psychiatric effects.
The most common offender is dextromethorphan (DXM), the cough suppressant found in many cold and flu products. At recommended doses, DXM rarely causes problems. But in overdose or deliberate abuse, it can produce hallucinations, dissociation, and psychosis. A review of DXM abuse noted that overdose symptoms can also be amplified by coformulated ingredients like antihistamines and decongestants that are bundled into the same pill or syrup.7PubMed. Dextromethorphan abuse: clinical effects and management When a person takes a large amount of a combination cold medicine, the hallucinations are almost certainly from the DXM, not the acetaminophen. But because the product label says “acetaminophen” on it, the association gets muddied.
A case report illustrates a different version of this problem. An elderly patient with very low body weight developed severe delirium after taking capsules that combined paracetamol with amantadine hydrochloride, a drug used for flu and sometimes Parkinson’s disease. Amantadine is known to cause confusion, hallucinations, and psychosis, especially in older or frail patients. Once the medication was stopped and the patient received supportive care, the psychiatric symptoms completely resolved within about 11 days.8PubMed Central. Severe Delirium Caused by Short-Term Administration of Compound Paracetamol and Amantadine Hydrochloride Capsules in an Elderly Patient With Extremely Low Body Weight In cases like these, the combination product gets blamed, and “paracetamol” appears in the case report, but the more pharmacologically plausible cause is the co-ingredient.
If you’ve experienced hallucinations or mental fogginess after taking an acetaminophen-containing product, check the full ingredient list. Diphenhydramine (Benadryl), which is included in some nighttime formulations, is another drug that can cause hallucinations in overdose or in sensitive individuals. The acetaminophen is unlikely to be the culprit when it is sharing a capsule with known psychoactive compounds.
High Doses and the Blood-Brain Barrier
At normal therapeutic doses, acetaminophen enters the brain in controlled amounts and does its work without overwhelming any particular system. At very high doses, something different may happen. A study in rats found that a single large dose of acetaminophen altered the integrity of the blood-brain barrier, the tightly sealed layer of cells that controls what gets into the brain from the bloodstream. Specifically, a marker molecule that normally stays outside the brain was able to leak through, and a co-administered opioid (codeine) accumulated in the brain at higher-than-expected levels. The researchers also found that a key structural protein in brain blood vessels was disrupted.9PubMed Central. High-Dose Acetaminophen Alters the Integrity of the Blood-Brain Barrier and Leads to Increased CNS Uptake of Codeine in Rats
The dose used in that study was very high, well above what a human would take therapeutically, but the finding has real-world relevance. People who intentionally or accidentally overdose on acetaminophen, or who chronically exceed recommended doses, could be letting more of other substances into their brains than the body’s defenses would normally allow. If someone takes too much of a product combining acetaminophen with an opioid or another psychoactive ingredient, the acetaminophen overdose may worsen the neurological effects of the other drug by physically opening the gate wider. Hallucinations in an overdose scenario could therefore be partly enabled by acetaminophen’s effect on the barrier, even if acetaminophen is not directly causing them.
Delirium in Older Adults
Hallucinations sometimes occur as part of delirium, a state of acute confusion that is common in hospitalized older adults, especially after surgery. Delirium can involve visual hallucinations, paranoia, agitation, and disorientation. Many drugs can trigger or worsen delirium, so the question of whether acetaminophen contributes to it or protects against it is clinically meaningful.
The evidence actually leans in acetaminophen’s favor here. Because acetaminophen can reduce the need for opioid painkillers after surgery, and because opioids are a well-established cause of delirium, using acetaminophen as part of a pain management plan may lower delirium risk. A randomized trial involving older patients undergoing cardiac surgery found that intravenous acetaminophen reduced delirium by roughly two-thirds compared to controls. A separate retrospective study of geriatric hip fracture patients found delirium rates were about half as high when acetaminophen was given intravenously rather than orally.10PubMed Central. Intravenous acetaminophen for postoperative delirium in older patients recovering from major non-cardiac surgery: a randomised-controlled study protocol The thinking is that acetaminophen achieves adequate pain control with less reliance on opioids, and this opioid-sparing effect is what drives the delirium reduction. Additionally, some researchers believe acetaminophen has anti-neuroinflammatory properties that could independently protect against delirium.
That said, the amantadine combination case mentioned earlier is a reminder that frail, underweight, or very old patients can react unpredictably to medications, particularly combinations. Context matters enormously. The same drug that appears protective in a controlled surgical setting could theoretically contribute to confusion in a very different clinical scenario, such as polypharmacy in a nursing home patient with multiple comorbidities.
Emotional Blunting and Altered Perception
Acetaminophen won’t make you see things that aren’t there at standard doses, but there is solid experimental evidence that it changes how you perceive and respond to the emotional world around you. These effects are not hallucinations, but they are a form of altered perception that most people taking two Tylenol for a headache would not expect.
In one study, participants who took acetaminophen rated the pain of other people as less severe and felt less personal distress when observing someone else in pain, compared to those who took a placebo. The drug appeared to dampen the affective (emotional) dimension of empathy without changing cognitive understanding of the situation.11PubMed Central. From painkiller to empathy killer: acetaminophen (paracetamol) reduces empathy for pain A separate experiment extended this finding to positive emotions: people who took acetaminophen felt less personal pleasure and less empathic warmth when reading about good things happening to others. Their ability to recognize that the scenarios were positive was unaffected, but the emotional resonance was turned down.12PubMed Central. A Social Analgesic? Acetaminophen (Paracetamol) Reduces Positive Empathy
These findings fit neatly with the neurochemistry described earlier. If acetaminophen acts on serotonin, cannabinoid, and related systems in the brain, it makes sense that it would dampen emotional reactivity alongside physical pain. The practical takeaway is that acetaminophen is not psychologically neutral. It softens your emotional responses, both negative and positive, even at the dose you’d take for a tension headache. Whether someone interprets that emotional dampening as “feeling weird” or “feeling off” depends on the person and the context, but it is a real, measurable neuropsychological effect that most people attribute to the drug itself.
When to Be Concerned
If you or someone you know experiences hallucinations, confusion, or unusual behavior after taking acetaminophen, there are a few things to consider before blaming the drug itself.
- Check all ingredients: If the product is a combination medication (cold and flu remedies, sleep aids, prescription painkillers), another ingredient is much more likely responsible. Dextromethorphan, diphenhydramine, and opioids are all more strongly linked to hallucinations than acetaminophen.
- Consider the dose: At recommended doses, acetaminophen is one of the safest pain relievers available. Overdose changes the equation, both through direct toxicity and, as the rat study suggests, through effects on the blood-brain barrier that could amplify other substances.
- Account for age and frailty: Older adults, especially those with low body weight or multiple medications, are at higher risk for delirium and neuropsychiatric drug reactions in general. Even though acetaminophen may reduce postoperative delirium, any medication change in a frail elderly person warrants monitoring.
- Rule out the underlying illness: Fever, infection, dehydration, and sleep deprivation can all cause hallucinations or delirium on their own. If someone is taking acetaminophen for a high fever and then experiences confusion, the fever itself is a much more likely explanation.
The pharmacovigilance signal exists, and it would be wrong to say acetaminophen has zero neuropsychiatric footprint given what we know about its brain activity. But the evidence for acetaminophen directly causing hallucinations at therapeutic doses in otherwise healthy people is essentially nonexistent. The rare reports that do surface almost always involve overdose, combination products, extreme age or frailty, or confounding illness.
How Acetaminophen Compares to Other Over-the-Counter Pain Relievers
It is reasonable to wonder whether ibuprofen or other common painkillers carry the same theoretical neuropsychiatric risk. A large-scale analysis of ibuprofen’s side effect profile found that central nervous system symptoms occurred in roughly 1 to 2 percent of users, a rate that was comparable to placebo in the same studies.13Pharmacotherapy: The Journal of Human Pharmacology and Drug Therapy. Nonprescription Ibuprofen: Side Effect Profile Those symptoms were things like dizziness and headache, not hallucinations. NSAIDs as a class have their own rare neuropsychiatric case reports, including aseptic meningitis and confusion, but these are considered extremely uncommon.
Acetaminophen’s deeper engagement with brain neurotransmitter systems, particularly serotonin and the endocannabinoid system, does make it neurochemically distinct from ibuprofen and similar anti-inflammatory drugs, which primarily work on inflammation pathways in the peripheral body. Whether that translates into a meaningful practical difference in neuropsychiatric risk at normal doses is unclear. For most people most of the time, both classes of over-the-counter pain relievers are safe and unremarkable from a psychiatric standpoint. The differences in brain activity are more relevant to researchers trying to understand these drugs’ full mechanism of action than to someone deciding what to take for a sore back.