Halothane is a non-flammable volatile anesthetic that was introduced in the mid-1950s and quickly became one of the most widely used inhaled agents for general anesthesia around the world. Its decline came gradually, driven primarily by accumulating evidence that it could cause a rare but potentially fatal form of liver damage, along with cardiovascular risks and the arrival of safer alternatives like sevoflurane and isoflurane. The story of halothane is really the story of modern anesthesiology growing up, learning to replace a drug that solved enormous problems with ones that introduced fewer new ones.
A Drug That Solved an Urgent Problem
Before halothane arrived, the operating room was a surprisingly dangerous place for reasons that had nothing to do with surgery itself. The dominant inhaled anesthetics of the era, ether and cyclopropane, were flammable. With the growing use of electrically powered equipment in operating theaters, the risk of fires and explosions was a genuine occupational hazard. Chloroform, the other major option, avoided the flammability problem but carried its own serious baggage: cardiac arrhythmias and liver damage. Anesthesiologists in the 1940s and early 1950s were effectively choosing between agents that could catch fire and agents that could stop the heart.
Halothane’s introduction in 1955 was considered a major breakthrough precisely because it was non-flammable while still being an effective, potent anesthetic.1PubMed. First use of halothane in the United States, C. Ronald Stephen, M.D. (1916-2006) It smelled relatively pleasant compared to ether, patients tolerated breathing it in more easily, and it provided smooth, controllable anesthesia. Within a few years, halothane had become the default volatile anesthetic in much of the world. For roughly two decades, it was the agent that most people undergoing surgery would breathe.
How Halothane Produces Unconsciousness
Halothane works largely by amplifying the brain’s own inhibitory signaling system. The key player is the GABA-A receptor, a protein on nerve cells that, when activated, quiets neural activity. Under normal conditions, the neurotransmitter GABA binds briefly to these receptors, opens a channel that lets chloride ions flow in, and the nerve cell calms down. Halothane makes this process more powerful in several ways.
At clinical concentrations, halothane enhances the ability of GABA to open chloride channels and also increases GABA’s binding affinity at the receptor, meaning it takes less of the neurotransmitter to produce the same inhibitory effect.2PubMed Central. Effects of halothane on GABA(A) receptor kinetics: evidence for slowed agonist unbinding Research on the receptor’s behavior showed that halothane slows the rate at which GABA detaches from its receptor, effectively prolonging each inhibitory signal at the synapse. At higher concentrations, halothane can even directly open the chloride channel without GABA being present at all, essentially mimicking the neurotransmitter on its own.2PubMed Central. Effects of halothane on GABA(A) receptor kinetics: evidence for slowed agonist unbinding The cumulative result is a widespread dampening of brain activity, producing unconsciousness, muscle relaxation, and suppression of reflexes.
Interestingly, halothane appears to act on the brain somewhat differently from the newer volatile anesthetics that eventually replaced it. A comparative study of cortical brain activity in rats found that halothane caused less overall depression of electrical activity than isoflurane, sevoflurane, or desflurane at equivalent anesthetic depths. The newer agents all produced burst-suppression patterns on EEG recordings, while halothane did not at any concentration tested.3PubMed Central. Comparative effects of halothane, isoflurane, sevoflurane and desflurane on the electroencephalogram of the rat This suggests that halothane’s mechanism has some fundamentally different character from its successors, even though the end result for the patient is similar.
The Liver Problem
The main reason halothane fell from favor is its association with liver injury, a complication that comes in two distinct forms. The first is mild and common: a slight, transient elevation in liver enzymes that occurs in a sizable fraction of patients after halothane anesthesia and resolves on its own without anyone noticing. The second is rare but terrifying: fulminant hepatitis that can destroy the liver and kill the patient.
The mechanism behind severe halothane hepatitis involves the body’s own processing of the drug. When halothane is metabolized in the liver, a specific enzyme (cytochrome P450 2E1) oxidizes it, producing a reactive chemical intermediate called trifluoroacetyl halide.4PubMed. Human hepatocytes express trifluoroacetylated neoantigens after in vitro exposure to halothane This intermediate can latch onto proteins on the surface of liver cells, chemically altering them. In most people, the immune system ignores these modified proteins. But in a small subset of patients, the immune system recognizes these trifluoroacetylated liver proteins as foreign invaders and mounts an attack against the patient’s own liver tissue.5PubMed. Halogenated anesthetics form liver adducts and antigens that cross-react with halothane-induced antibodies
In vitro work confirmed that P450 2E1 is the principal enzyme responsible for this oxidative pathway, with inhibitors of this enzyme blocking the formation of trifluoroacetic acid and bromide by roughly 85 to 90 percent.6ScienceDirect. Cytochrome P450 2E1 is the Principal Catalyst of Human Oxidative Halothane Metabolism in Vitro The efficiency of this enzyme matters because more oxidative metabolism means more reactive intermediate, which means more altered liver proteins and a greater chance of triggering the immune response.
Who Was Most at Risk
Halothane hepatitis was never a random event. Research identified a constellation of factors that made certain patients far more vulnerable, and the pattern was consistent across both human case analyses and animal models.
The strongest risk factor was repeated exposure. Most cases of severe halothane hepatitis occurred in patients who had received halothane anesthesia more than once, because the initial exposure could sensitize the immune system to those trifluoroacetylated liver proteins, setting the stage for a far more aggressive response the next time around.7PubMed Central. Halothane-induced hepatitis: A forgotten issue in developing countries: Halothane-induced hepatitis Other identified risk factors included female sex, adult age (children were less susceptible), genetic predisposition, fasting, and conditions that reduced blood oxygen supply to the liver.8PubMed. Risk factors for halothane hepatitis
Mouse model experiments reinforced the clinical picture. Female BALB/cJ mice developed severe liver injury from halothane while males showed milder damage. Fasting increased vulnerability. Younger animals were more resistant than adults. And the entire effect was strain-dependent: C57BL/6 mice were essentially insensitive to halothane-induced liver damage, pointing to a strong genetic component.9PubMed Central. A mouse model of severe halothane hepatitis based on human risk factors The fact that so many risk factors had to align meant that halothane hepatitis was rare in the absolute sense, but when it struck, the mortality rate was high enough to make anesthesiologists deeply uncomfortable with the drug.
Cardiac Risks and Arrhythmias
Liver toxicity drove the headlines, but halothane had a second major liability: it sensitized the heart to catecholamines, particularly epinephrine. During surgery, a patient’s body naturally releases catecholamines in response to stress and pain, and surgeons sometimes inject epinephrine locally to control bleeding. Under halothane anesthesia, these catecholamines could trigger dangerous heart rhythm disturbances.
Animal studies demonstrated that epinephrine given intravenously during halothane anesthesia readily produced ventricular arrhythmias, including bigeminy and multifocal ventricular tachycardia.10American Heart Journal. The mechanism of sensitization of the ventricle to epinephrine by halothane The interaction appeared related to changes in heart rate: even very brief increases in rate caused prolonged periods of abnormal rhythm, suggesting that halothane fundamentally altered the electrical properties of heart muscle in a way that promoted dangerous re-entry circuits.11PubMed. Halothane-epinephrine-induced cardiac arrhythmias and the role of heart rate
In practice, this meant that anesthesiologists using halothane had to be extremely cautious about the total dose of local epinephrine a surgeon could infiltrate. Strict limits were enforced, and any sign of ectopic heartbeats required immediate action. The newer agents that replaced halothane are much less prone to this particular interaction, which eliminated one of the more stressful aspects of keeping patients safe during surgery.
Malignant Hyperthermia
Halothane is one of the volatile anesthetics capable of triggering malignant hyperthermia, a rare but life-threatening emergency in genetically susceptible individuals. During an episode, skeletal muscles undergo uncontrolled contraction, body temperature skyrockets, and without rapid treatment the condition can be fatal.
The underlying problem involves a calcium release channel in muscle cells called the ryanodine receptor (RyR1). In people carrying certain mutations in this receptor, halothane causes the channel to open inappropriately, flooding muscle cells with calcium and triggering sustained contraction. Research found that halothane caused significantly larger elevations of calcium inside muscle fibers from susceptible animals compared to normal ones, and it shifted the voltage at which calcium release was activated in a direction that made release much easier to trigger at the muscle’s normal resting state.12PubMed Central. Voltage modulates halothane-triggered Ca2+ release in malignant hyperthermia-susceptible muscle In other words, halothane effectively lowers the threshold for the mutated receptor to dump calcium where it should not.
The susceptibility depends on the specific RyR1 mutation involved. Different mutations make the receptor more sensitive to different triggers, and contracture test results vary depending on the patient’s underlying genetics.13PubMed. Results of contracture tests with halothane, caffeine, and ryanodine depend on different malignant hyperthermia-associated ryanodine receptor gene mutations This isn’t unique to halothane: all modern volatile anesthetics can trigger malignant hyperthermia in susceptible individuals, and the treatment (dantrolene) is the same regardless of the triggering agent. But halothane’s historical role in identifying and studying malignant hyperthermia gave it an outsized association with the condition in clinical lore.
The Agents That Replaced It
Halothane’s decline was not a sudden event but a gradual displacement by newer volatile anesthetics, mainly isoflurane (introduced in the 1980s), sevoflurane (1990s), and desflurane (1990s). Each addressed one or more of halothane’s liabilities while preserving the core function of providing controllable inhaled anesthesia.
Sevoflurane became the most prominent replacement for halothane in situations where the older drug had been especially valued, particularly in pediatric anesthesia and mask inductions (where a patient breathes the anesthetic through a face mask rather than having it delivered through an IV). Head-to-head comparisons showed that sevoflurane produced faster induction, shorter intubation times, better hemodynamic stability, and fewer airway complications like coughing, breath-holding, and excessive salivation.14PubMed Central. Sevoflurane versus halothane for induction of anesthesia in pediatric and adult patients Recovery was also faster: patients regained consciousness sooner and could be discharged from the recovery room earlier.
In children specifically, sevoflurane shortened recovery and discharge times compared to halothane, though researchers noted that the faster emergence came with a higher incidence of postoperative agitation.15PubMed. Effects of sevoflurane anaesthesia on recovery in children: a comparison with halothane This agitation, sometimes called emergence delirium, became a recognized side effect of sevoflurane that pediatric anesthesiologists learned to manage with supplemental medications. It was considered a vastly preferable trade-off compared to halothane’s hepatotoxicity risk.
None of the newer agents produce the trifluoroacetylated liver protein adducts that cause halothane hepatitis. They are also significantly less likely to sensitize the heart to catecholamines. Sevoflurane, isoflurane, and desflurane are all still capable of triggering malignant hyperthermia in susceptible individuals, so that particular risk was not eliminated by the switch, but the hepatic and cardiac liabilities were essentially removed from the equation.
Where Halothane Persisted
In wealthy countries, halothane was largely phased out of human anesthesia by the late 1990s and early 2000s. But the drug continued to see widespread use in low- and middle-income countries for years afterward, and in some places it has remained in service far longer than many Western anesthesiologists realize.
The reasons are practical. Halothane is cheap. It works reliably with simple, low-tech vaporizer systems, including draw-over apparatus that do not require compressed gas supplies or complex infrastructure.16International Surgery Journal. The discontinuation of halothane production and its implications for anaesthesia practice in resource-limited countries: a Nigerian perspective In settings where electricity and piped medical gases cannot be taken for granted, these qualities matter. A commentary in a WHO regional publication noted that halothane remained widely used in developing countries including Iran specifically because of its low price.17PubMed. Halothane: how should it be used in a developing country?
The discontinuation of halothane production by major manufacturers has created a more urgent situation in these settings. When the drug simply becomes unavailable, facilities that relied on it must transition to alternatives like sevoflurane or isoflurane, which are more expensive and may require different or upgraded delivery equipment. The transition is not just a matter of swapping one bottle for another. It can require training staff on new vaporizers, budgeting for higher drug costs, and in some cases upgrading anesthesia machines entirely. For hospitals operating on extremely tight margins, this forced transition presents real challenges to maintaining surgical capacity.
Halothane in Veterinary Medicine
Even as halothane disappeared from human operating rooms in high-income countries, it held on longer in veterinary practice. One area where it saw continued use was equine anesthesia, a field with its own set of challenges because horses are uniquely prone to complications under general anesthesia.
A large prospective trial comparing halothane and isoflurane in horses concluded that halothane remained an acceptable anesthetic for maintenance, though isoflurane appeared safer in young horses and high-risk cases.18PubMed. Is isoflurane safer than halothane in equine anaesthesia? Results from a prospective multicentre randomised controlled trial Veterinary anesthesia has generally followed the same trajectory as human anesthesia, with isoflurane and sevoflurane gradually replacing halothane, but the transition happened more slowly. Cost sensitivity is greater in veterinary practice, and the regulatory pressure to abandon older agents is less intense than in human medicine.
The Diagnostic Legacy of Halothane
Halothane has an afterlife that outlasts its clinical use as an anesthetic. The caffeine-halothane contracture test (CHCT) remains the standard diagnostic method for determining whether a person is susceptible to malignant hyperthermia. In this test, a small muscle biopsy is exposed to halothane and caffeine separately, and the contracture response is measured. An abnormally strong contraction in response to either substance indicates susceptibility.
Data from a Canadian malignant hyperthermia investigation unit collected over five years showed that of 121 patients tested, about half were normal, while the rest showed hypersensitivity to halothane alone or to both halothane and caffeine. Strikingly, all positive diagnoses in this group were based on an excessive response to halothane rather than caffeine, meaning the halothane portion of the test was driving every diagnosis.19British Journal of Anaesthesia. Abnormal calcium signalling and the caffeine–halothane contracture test The patients who reacted to halothane alone were, by a composite clinical index, actually more symptomatic than those who reacted to both agents.
This means that even as halothane vanishes from operating rooms globally, the drug itself needs to remain available for diagnostic laboratories that perform contracture testing. It is a peculiar situation: a drug largely abandoned for its intended purpose remains indispensable for identifying people who are vulnerable to a complication it helped define. Genetic testing for RyR1 mutations is increasingly available and can identify many susceptible individuals without a muscle biopsy, but the contracture test remains the gold standard when genetic results are inconclusive or when a mutation of uncertain significance is found. So long as that remains true, halothane will continue to be manufactured in small quantities for diagnostic use, a vestige of its former ubiquity.
Occupational Exposure in the Operating Room
One concern that rarely made it into the public conversation about halothane was the risk to the people administering it. Operating room staff breathe trace amounts of volatile anesthetics throughout their shifts, day after day, for entire careers. With halothane, this chronic low-level exposure raised concerns about potential health effects including liver damage, reproductive problems, and neurological symptoms.
Monitoring and controlling occupational exposure to inhaled anesthetics became a recognized concern in anesthesia practice.20Springer Link. Field application of SPME as a novel tool for occupational exposure assessment with inhalational anesthetics Scavenging systems, which capture exhaled anesthetic gases and vent them outside the building, became standard in modern operating rooms partly because of these concerns. The shift away from halothane was not motivated primarily by occupational health worries, but the broader transition to agents with lower toxicity profiles benefited staff as well as patients. In settings where halothane persisted longer and scavenging equipment was limited or absent, the occupational exposure concern remained more acute.