The Link Between Anticholinergics and Body Temperature
Anticholinergic medications work by blocking the neurotransmitter acetylcholine [1.3.2]. Acetylcholine plays a crucial role in the autonomic nervous system, which controls involuntary bodily functions, including the heat-regulation response of sweating [1.3.2]. When these drugs block acetylcholine's effects on the sweat glands, they impair or prevent perspiration, a condition known as anhidrosis [1.3.4, 1.6.2]. Sweating is the body's primary mechanism for dissipating heat; without it, core body temperature can rise, particularly during exposure to hot weather, strenuous exercise, or even a hot bath [1.2.2, 1.3.4]. A 2024 systematic review and meta-analysis found that drugs with strong anticholinergic properties increased core temperature by an average of 0.42°C in air temperatures of 30°C or higher [1.2.1]. This impaired thermoregulation can lead to dangerous conditions like heat exhaustion, hyperthermia (a dangerously overheated body), and potentially fatal heat stroke [1.2.2, 1.8.1].
Understanding Anticholinergic-Induced Hyperthermia
Anticholinergic-induced hyperthermia occurs primarily because the body cannot effectively lose heat through sweating [1.3.3, 1.3.4]. The hypothalamus, the brain's thermostat, normally triggers sweating to cool the body down [1.3.2]. Anticholinergics interfere with this signal at the peripheral muscarinic receptors in sweat glands [1.3.4]. In cases of anticholinergic toxicity, this inability to sweat is a key diagnostic feature, often presenting as hot, red, and dry skin [1.3.1, 1.5.4]. The classic mnemonic for anticholinergic toxidrome includes "hot as a hare," directly referencing this hyperthermic effect [1.5.1, 1.5.2]. While therapeutic doses might not significantly alter temperature in neutral environments, the risk escalates dramatically in hot conditions, where body temperature can rise precipitously once sweating stops [1.6.2].
Who is Most at Risk?
Certain populations are more vulnerable to the thermoregulatory effects of anticholinergics:
- Older Adults: The elderly are particularly susceptible due to age-related changes in physiology, a decreased cholinergic reserve, and often being on multiple medications (polypharmacy) [1.5.5, 1.9.1]. They have a reduced capacity to thermoregulate, and the addition of an anticholinergic drug further increases their risk of falls, delirium, and heat-related illness [1.8.5, 1.9.2].
- Infants and Children: Children have immature thermoregulation mechanisms and are more likely to develop temperature disturbances from these medications [1.3.3, 1.6.3].
- Individuals with Chronic Illnesses: Patients with cardiovascular disease, psychiatric disorders, or Parkinson's disease are often prescribed medications with anticholinergic effects and may have a compromised ability to handle heat stress [1.2.1, 1.2.3].
- People Taking Multiple Medications: The cumulative effect of several drugs with anticholinergic properties, known as the "anticholinergic burden," significantly increases the risk [1.9.1].
Common Medications with Anticholinergic Effects
Many prescription and over-the-counter (OTC) drugs possess anticholinergic properties. It's not just drugs formally classified as anticholinergics; others like some antidepressants and antihistamines also carry these effects [1.9.1].
- First-Generation Antihistamines: Diphenhydramine (found in Benadryl, Tylenol PM, Advil PM) and Dimenhydrinate (Dramamine) are common examples [1.4.1, 1.4.4, 1.8.4].
- Tricyclic Antidepressants: This class of drugs is known for anticholinergic side effects [1.4.5].
- Overactive Bladder Medications: Drugs like oxybutynin (Ditropan XL) and tolterodine (Detrol) are prescribed to treat urinary incontinence [1.4.4].
- Antipsychotics: Medications such as olanzapine and clozapine can have anticholinergic effects [1.4.5, 1.8.1].
- Muscle Relaxants: Orphenadrine is an example of a muscle relaxant with anticholinergic properties [1.4.3].
- Anti-Parkinson's Agents: Drugs like benztropine (Cogentin) are used to treat Parkinson's disease [1.4.2, 1.4.4].
Comparison of Heat-Related Syndromes
It is important to distinguish anticholinergic hyperthermia from other drug-induced syndromes.
| Feature | Anticholinergic Hyperthermia | Serotonin Syndrome | Neuroleptic Malignant Syndrome (NMS) |
|---|---|---|---|
| Primary Mechanism | Impaired sweating (anhidrosis) from muscarinic receptor blockade [1.6.2] | Excess serotonin activity [1.7.1] | Dopamine blockade [1.3.3] |
| Skin | Hot, flushed, DRY [1.3.1] | Diaphoretic (sweating) [1.7.3] | Diaphoretic (sweating) or variable [1.3.3] |
| Muscle Tone | Normal or increased, possible myoclonic jerking [1.5.4, 1.5.5] | Increased, especially clonus (involuntary muscle contractions) in lower extremities [1.7.3] | Severe "lead-pipe" rigidity [1.3.3] |
| Pupils | Dilated (mydriasis) [1.5.4] | Dilated (mydriasis) [1.7.3] | Normal or variable |
| Bowel Sounds | Decreased or absent [1.3.3] | Hyperactive, may include diarrhea [1.7.2] | Normal or decreased |
Management and Prevention
If anticholinergic-induced hyperthermia is suspected, the first step is to discontinue the offending medication(s) [1.7.4, 1.7.5]. Management focuses on supportive care and rapid cooling. This can include external cooling methods like misting with fans or applying ice packs [1.7.2, 1.7.5]. Unlike fevers caused by infection, drug-induced hyperthermia does not respond to standard antipyretics like acetaminophen or ibuprofen because the body's temperature set point in the hypothalamus is not altered [1.7.2, 1.7.3]. In severe cases of toxicity (anticholinergic toxidrome), hospitalization is required for aggressive cooling, sedation with benzodiazepines, and potentially the administration of an antidote called physostigmine [1.5.1, 1.7.2].
Prevention is key for individuals taking these medications. Patients should be counseled on the risks, especially during hot weather [1.6.1]. Key strategies include staying hydrated, avoiding high-temperature environments, seeking air-conditioning, and being aware of the symptoms of heat exhaustion and heat stroke [1.2.4].
Conclusion
Yes, anticholinergics unequivocally increase body temperature. They do so by blocking the body's ability to sweat, thereby compromising its primary cooling system [1.2.2, 1.3.2]. This effect elevates the risk of developing potentially life-threatening hyperthermia and heat stroke, particularly in vulnerable populations and during periods of heat stress [1.8.1, 1.8.5]. Awareness of which medications carry anticholinergic properties and understanding the signs of heat-related illness are crucial for safely using these drugs. Patients should never stop a prescribed medication without consulting their doctor but should discuss the risks and strategies for staying safe in the heat [1.2.4, 1.8.5].
Disclaimer: This article is for informational purposes only and does not constitute medical advice. Always consult with a healthcare professional before making any decisions about your medication. For more information on medication safety in the heat, you can visit the CDC's guidance for clinicians.
