Adrenaline (epinephrine) is a chemical messenger produced in the adrenal glands that functions as both a hormone and a medication. This compound, part of the catecholamine family, is swiftly released into the bloodstream to prepare the body for immediate, high-energy demands. Its primary biological function involves triggering widespread physiological changes by binding to specific receptors on cells throughout the body.
Adrenaline’s Natural Function
Adrenaline is released from the adrenal medulla in response to perceived stress, danger, or excitement, initiating a rapid systemic mobilization of energy. This release causes a sudden increase in the force and speed of heart contractions, pumping blood more efficiently to the working muscles. Simultaneously, it stimulates the liver to break down stored glycogen into glucose, releasing a quick supply of fuel into the bloodstream.
The hormone also influences the circulatory system by causing peripheral vasoconstriction, narrowing blood vessels in areas like the skin and gut. This redirects blood toward the brain, heart, and skeletal muscles. The body tightly regulates this endogenous release, ensuring levels remain within a safe, naturally occurring range, typically below 1200 picomoles per liter (pmol/L) in non-stressed conditions.
Defining a Lethal Dose
Determining a precise lethal dose (LD50) of administered epinephrine for humans is ethically and medically impossible. Toxicity is highly dependent on a person’s underlying health, the route of administration, and the speed of injection. In animal studies, the concept of LD50—the dose causing death in 50% of the population—is used, but these figures do not translate directly to human risk.
For example, plasma concentrations of adrenaline in patients receiving high-dose therapeutic infusions for conditions like cardiac arrest can reach 10,000 to 100,000 nanograms per liter (ng/L). This is significantly higher than levels seen during natural stress, highlighting the individual variability in tolerance. A dose that may be life-saving when injected intramuscularly could be fatal if administered directly into a vein without careful dilution and monitoring.
How Excess Adrenaline Causes Harm
High levels of adrenaline, typically from external administration, overwhelm the body’s regulatory mechanisms and cause physiological damage, primarily to the cardiovascular system. Excessive stimulation of alpha-1 adrenergic receptors leads to extreme vasoconstriction and resulting high blood pressure. This sharp rise in pressure increases the risk of a cerebrovascular hemorrhage, or stroke.
Overstimulation of beta-1 adrenergic receptors in the heart causes an accelerated heart rate, or tachycardia, and a corresponding increase in myocardial oxygen demand. This demand can quickly outstrip the heart’s blood supply, leading to acute myocardial ischemia or infarction. The combination of widespread vasoconstriction and cardiac overstimulation can trigger fatal cardiac arrhythmias, such as ventricular tachycardia, or pulmonary edema due to the strain on the heart.
Medical Use and Safety Precautions
Adrenaline is a medication used in emergency settings for conditions such as anaphylaxis, where a rapid injection reverses airway closure and severe hypotension. The standard therapeutic dose for an adult in an auto-injector is 0.3 milligrams (mg) administered intramuscularly into the thigh. In a hospital setting, it is also used for cardiac arrest, where a dose of 1 mg is given intravenously every three to five minutes.
The drug has a narrow therapeutic window, meaning the difference between a therapeutic dose and a toxic one is small. For continuous intravenous infusions, such as those used to maintain blood pressure in septic shock, the dose is precisely titrated, often starting as low as 0.05 micrograms per kilogram per minute (mcg/kg/min). Accidental overdose or improper administration, such as injecting an auto-injector dose into a finger or directly into a vein, carries a high risk of localized tissue necrosis or systemic cardiovascular collapse.