Cholinergic vs Anticholinergic Drugs - Understanding cholinergic and anticholinergic drugs is a cornerstone of pharmacology, especially for nursing, medical, and pharmacy students. These two groups act on the autonomic nervous system (ANS) but have opposite effects—one stimulates it, while the other blocks it.
In this article, we’ll break down their mechanisms, effects, and differences using simple analogies and mnemonics like SLUDGE and BOATS, as illustrated in the image.
Overview of the Autonomic Nervous System (ANS)
The autonomic nervous system controls involuntary body functions such as heart rate, digestion, respiration, and glandular secretion. It has two primary divisions:
| Division | Full Form | Main Function | Neurotransmitter |
|---|---|---|---|
| PNS | Parasympathetic Nervous System | “Rest and Digest” | Acetylcholine (ACh) |
| SNS | Sympathetic Nervous System | “Fight or Flight” | Epinephrine / Norepinephrine |
Cholinergic and anticholinergic drugs act primarily on acetylcholine receptors, thereby influencing these autonomic pathways.
What Are Cholinergic Drugs?
Cholinergic drugs (also called parasympathomimetics) mimic the effects of acetylcholine (ACh) and stimulate the parasympathetic nervous system (PNS).
Think of them as drugs that help the body “rest and digest.”
They activate muscarinic and nicotinic receptors, leading to increased glandular secretion, smooth muscle contraction, and relaxation of sphincters.
Mnemonic for Cholinergic Effects: “SLUDGE”
To easily remember the adverse or excessive effects of cholinergic drugs, use the mnemonic S-L-U-D-G-E:
| Letter | Effect | Description |
|---|---|---|
| S | Salivation | Excessive drooling or saliva production |
| L | Lacrimation | Increased tear secretion |
| U | Urination | Overactive bladder or frequent urination |
| D | Diaphoresis | Excessive sweating |
| G | GI Distress | Abdominal cramps or diarrhea |
| E | Emesis | Nausea or vomiting |
💡 Tip for students:
Think of “COLON” when you think Cholinergic, because these drugs activate gut and glandular functions.
Mechanism of Action
Cholinergic drugs work by:
1. Directly stimulating cholinergic receptors (e.g., Bethanechol, Pilocarpine).Examples of Cholinergic Drugs
| Drug Name | Type | Therapeutic Use |
|---|---|---|
| Bethanechol | Direct-acting | Urinary retention |
| Pilocarpine | Direct-acting | Glaucoma |
| Neostigmine | Indirect-acting | Myasthenia gravis |
| Physostigmine | Indirect-acting | Anticholinergic poisoning |
| Donepezil | Indirect-acting | Alzheimer’s disease |
Clinical Applications of Cholinergics
- Ophthalmology: Constrict pupils in glaucoma.
- Gastrointestinal: Stimulate bowel movement in postoperative ileus.
- Urinary system: Treat urinary retention.
- Neurology: Improve muscle tone in myasthenia gravis.
Adverse Effects
Due to overstimulation of the PNS, cholinergic drugs may cause:
- Bradycardia
- Hypotension
- Bronchoconstriction
- Diarrhea and cramps
- Excessive salivation and sweating
In extreme cases, cholinergic crisis may occur — a medical emergency with respiratory muscle weakness.
What Are Anticholinergic Drugs?
Anticholinergic drugs (also called parasympatholytics) block acetylcholine (ACh) at muscarinic receptors, inhibiting the parasympathetic nervous system (PNS) and allowing the sympathetic system (SNS) to dominate.
In short, these drugs make your body “fight or flight.”
They act as bronchodilators, mydriatics, and smooth muscle relaxants.
Mnemonic for Anticholinergic Effects: “ANTI-COLON” or “ANTI-GUTS”
To remember their adverse or physiological effects, use GUTS:
| Letter | Effect | Explanation |
|---|---|---|
| G | GI Motility Reduced | Constipation due to reduced peristalsis |
| U | Urine Retention | Relaxation of detrusor muscle |
| T | Tachycardia | Increased heart rate |
| S | Sight Impaired | Dilated pupils and blurred vision |
💡 Tip for learners:
“ANTI-COLON” means opposite of cholinergic effects — dry mouth, constipation, blurred vision, and urinary retention.
Mnemonic for Common Anticholinergic Drugs: “BOATS”
| Letter | Drug | Use |
|---|---|---|
| B | Benztropine | Parkinson’s disease (reduces tremors) |
| O | Oxybutynin | Overactive bladder |
| AT | Atropine | Bradycardia, pre-anesthesia drying agent |
| S | Scopolamine | Motion sickness, nausea |
Mechanism of Action
Anticholinergics block muscarinic acetylcholine receptors in smooth muscles, secretory glands, and the CNS.
This reduces parasympathetic stimulation, resulting in dilated pupils, dry mouth, and increased heart rate.
Therapeutic Applications of Anticholinergic Drugs
| System | Clinical Use | Example Drug |
|---|---|---|
| Respiratory | Bronchodilation in asthma/COPD | Ipratropium, Tiotropium |
| Cardiac | Increases heart rate in bradycardia | Atropine |
| GI System | Reduces motion sickness and GI spasms | Scopolamine |
| Urinary | Relieves urinary incontinence | Oxybutynin |
| CNS | Treats Parkinson’s disease | Benztropine |
Adverse Effects of Anticholinergics
Due to the inhibition of parasympathetic activity, side effects can be remembered as “Dry as a bone, blind as a bat, hot as a hare, mad as a hatter.”
| Effect | Explanation |
|---|---|
| Dry mouth | Reduced salivation |
| Blurred vision | Pupil dilation |
| Constipation | Decreased GI motility |
| Tachycardia | Heart rate increases |
| Confusion / Hallucinations | CNS toxicity, especially in elderly |
Comparison Table: Cholinergic vs. Anticholinergic
| Feature | Cholinergic Drugs | Anticholinergic Drugs |
|---|---|---|
| Main Action | Stimulate PNS (“Rest & Digest”) | Block PNS (“Fight or Flight”) |
| Receptor Type | Muscarinic & Nicotinic | Muscarinic (ACh) |
| Effect on Eyes | Pupil constriction (miosis) | Pupil dilation (mydriasis) |
| Effect on Heart | Slows heart rate (bradycardia) | Increases heart rate (tachycardia) |
| Effect on GI Tract | Stimulates motility | Decreases motility |
| Effect on Lungs | Bronchoconstriction | Bronchodilation |
| Effect on Bladder | Promotes urination | Causes urine retention |
| Salivation | Increases | Decreases |
| Mnemonic | SLUDGE | BOATS / GUTS |
| Example Drugs | Bethanechol, Neostigmine | Atropine, Scopolamine |
Mnemonic Summary
For Cholinergics
S-L-U-D-G-E → Salivation, Lacrimation, Urination, Diaphoresis, GI Distress, Emesis.
For Anticholinergics
B-O-A-T-S → Benztropine, Oxybutynin, Atropine, Scopolamine.
Clinical Significance
- In anesthesia: Atropine is used preoperatively to dry secretions.
- In poisoning: Physostigmine (cholinergic) reverses anticholinergic toxicity.
- In chronic diseases: Anticholinergics relieve symptoms in COPD and Parkinson’s disease.
- In eye care: Pilocarpine (cholinergic) helps in glaucoma, while tropicamide (anticholinergic) aids in pupil dilation for eye exams.
| Aspect | Cholinergic Drugs | Anticholinergic Drugs |
|---|---|---|
| System Activated | Parasympathetic (PNS) | Sympathetic (SNS) |
| Primary Action | Rest and Digest | Fight or Flight |
| Mnemonic | SLUDGE | BOATS |
| Key Examples | Bethanechol, Neostigmine | Atropine, Scopolamine |
| Common Uses | Glaucoma, Myasthenia gravis | COPD, Motion sickness |
| Antidote Relationship | Physostigmine reverses anticholinergic overdose | Atropine treats cholinergic overdose |
FAQ
1. What is the main difference between cholinergic and anticholinergic drugs?
Cholinergic drugs stimulate the parasympathetic nervous system, while anticholinergics inhibit it, producing opposite effects.
2. Why are cholinergic drugs called “parasympathomimetics”?
Because they mimic the effects of the parasympathetic neurotransmitter acetylcholine.
3. What happens in anticholinergic toxicity?
Symptoms include dry mouth, blurred vision, urinary retention, hallucinations, and tachycardia — collectively known as anticholinergic syndrome.
4. Which drug is used as an antidote for anticholinergic poisoning?
Physostigmine, a cholinesterase inhibitor, increases acetylcholine levels and reverses toxicity.
5. Can cholinergic and anticholinergic drugs be used together?
Generally, no — they have opposite mechanisms and can neutralize each other’s effects, though both may appear in complex clinical protocols under strict supervision.
