When illness strikes, the experience often goes beyond physical aches and pains, bringing with it a profound sense of mental alteration that many describe as feeling “out of it” or “on drugs.” This sensation is not a psychological reaction to discomfort but a true physiological response to the body’s battle against an invader. The feeling can range from persistent cognitive fog and difficulty concentrating to overwhelming fatigue and altered sensory input. This temporary state is a biologically driven phenomenon resulting from the immune system’s communication with the brain.
The Body’s Immune Response and the Brain
The primary cause of this mental disruption is the body’s immediate inflammatory response to infection. When a virus or bacteria enters, immune cells release signaling molecules called pro-inflammatory cytokines, which coordinate the defense strategy. These inflammatory signals communicate with the central nervous system through several pathways, despite the protective blood-brain barrier. Cytokines can cross the barrier or activate cells lining the brain’s blood vessels to produce their own inflammatory molecules.
A faster communication route involves activating the vagus nerve, which acts as a direct neural highway to the brainstem. Once these signals reach the brain, they disrupt normal neurotransmitter function and neural circuitry. The inflammatory signals alter the metabolism of neurotransmitters like serotonin and dopamine, which are crucial for regulating mood, motivation, and cognition. This neuro-immune crosstalk leads to a temporary state of brain dysfunction, manifesting as cognitive slowing and mental cloudiness.
Understanding Sickness Behavior
The symptoms of feeling mentally altered are part of a coordinated, evolutionarily conserved strategy known as “sickness behavior.” This behavioral shift is an organized defense strategy designed to maximize the body’s resources for fighting the infection. The goal of this state is to conserve energy that would otherwise be spent on activities like foraging or social interaction.
Sickness behavior encompasses motivational and behavioral changes, including lethargy, social withdrawal, loss of appetite, and anhedonia (the inability to feel pleasure). The cognitive slowing and difficulty concentrating are also components of this adaptive response. By making the individual feel unwell and unmotivated, the body ensures metabolic energy is redirected to the immune system to fuel the fever and the production of immune cells. This state temporarily reorganizes priorities, forcing a period of rest and recovery.
When High Fever Causes Altered Perception
While sickness behavior causes general cognitive fog, a high fever can lead to more extreme sensations of altered perception, such as delirium or hallucinations. A significant elevation in core body temperature, known as hyperthermia, acutely disrupts normal brain function because neurons are highly sensitive to temperature fluctuations. Temperatures above the normal range interfere with cellular metabolism and the integrity of neuronal membranes.
This disruption causes acute encephalopathy, a temporary brain disorder leading to confusion, severe disorientation, and fluctuating consciousness. The “on drugs” sensation often describes this delirious state, which may include visual or auditory hallucinations and altered spatial awareness. While this is generally a temporary phenomenon, it is a sign of severe systemic stress. Delirium with high fever warrants medical attention, especially if accompanied by other severe symptoms.
Common Non-Infection Related Contributors
The feeling of mental alteration can be compounded or caused by factors separate from the immune response itself. One major contributor is the state of hydration, which is often compromised during illness due to fever, vomiting, or diarrhea. Even mild dehydration can impair cognitive function, leading to fatigue, mood changes, and difficulty with attention and memory. When the body lacks sufficient fluid, cerebral blood flow can be reduced, and electrolyte imbalances occur, which profoundly affect brain cell function. This lack of mental clarity, or “brain fog,” adds to the feeling of being disoriented and unwell.
Over-the-counter (OTC) cold and flu remedies can also contribute to a foggy mental state. Many common multi-symptom medications contain first-generation antihistamines, such as diphenhydramine. These older drugs are lipophilic, meaning they easily cross the blood-brain barrier. Once across, they block the neurotransmitter acetylcholine, which is crucial for memory and attention. Blocking acetylcholine causes side effects like drowsiness, impaired concentration, and confusion, closely mimicking a “drugged” feeling.