Atropine

BRAND NAMES

• AtroPen: AtroPen is an auto-injector designed for the treatment of poisoning caused by specific insecticides and nerve agents.

• Atropisol: An eye drop solution used for ophthalmic purposes.

• Isopto Atropine: Available as either an eye drop solution or ointment for use in the eyes.

MECHANISM OF ACTION

Atropine works by binding to and blocking muscarinic acetylcholine receptors, competitively inhibiting the effects of acetylcholine and related compounds. It functions as a reversible, non-selective antagonist across all five muscarinic receptor subtypes—M1 through M5. By doing so, atropine counteracts the actions of acetylcholine on tissues influenced by postganglionic cholinergic nerves, including smooth muscle, cardiac tissue, exocrine glands, and the central nervous system. It also affects less innervated smooth muscle that still responds to naturally occurring acetylcholine. The effects of atropine can be reversed by increasing acetylcholine levels at receptor sites, such as through the use of anticholinesterase drugs that prevent acetylcholine breakdown.

PHARMACOKINETICS:

Absorption

Atropine is efficiently absorbed through various routes, including oral, intramuscular, and intravenous administration. However, the rate and extent of absorption can vary considerably depending on the route used.

Distribution

After administration, atropine distributes quickly and extensively throughout the body, reaching tissues and fluids such as the central nervous system (CNS), eyes, and placenta. This wide distribution is attributed to its lipophilic (fat-soluble) nature.

Metabolism

Atropine is primarily metabolized in the liver through enzymatic hydrolysis, producing major metabolites such as noratropine, atropine N-oxide, tropine, and tropic acid. Its metabolism can be inhibited by organophosphate pesticides. Around 13% to 50% of the drug is excreted unchanged in the urine.

Excretion

Atropine is eliminated from the body primarily through renal excretion. Following metabolism in the liver, both unchanged drug and its metabolites are excreted in the urine. Approximately 13% to 50% of atropine is excreted unchanged, while the remainder is eliminated as various metabolites. The rate of excretion can be influenced by factors such as urine pH, with acidic urine promoting faster elimination.

PHARMACODYNAMICS

Atropine is an antimuscarinic agent that blocks the effects of acetylcholine. In low doses, it can slow the heart rate, while higher doses may cause tachycardia by inhibiting vagal control. Compared to scopolamine, atropine has stronger and longer-lasting effects on the heart, intestines, and bronchial muscles but weaker effects on the eyes and some glands. It increases respiratory rate and depth, likely due to bronchodilation. Atropine can prevent or reverse vagally-induced bradycardia, asystole, and certain types of heart block, particularly when triggered by drugs like choline esters or anticholinesterases. Though it counters peripheral vasodilation caused by cholinergic agents, atropine alone has minimal and inconsistent effects on blood vessels and blood pressure. 

ADMINISTRATION

The method of atropine administration can vary significantly based on the medical condition being treated. It may be delivered through several routes, including intravenous (IV), intramuscular (IM), intraosseous (IO), or as ophthalmic eye drops. The following outlines the routes of administration according to specific therapeutic uses.

DOSAGE AND STRENGTH

Atropine is available in various forms—injectable, ophthalmic, oral, and nasal—with dosages and strengths varying based on the route, medical condition, and patient’s age. Injectable atropine is used for emergencies like bradycardia and organophosphate poisoning, with strengths such as 0.1 mg/mL to 1 mg/mL and autoinjectors (e.g., 2 mg/0.7 mL). Ophthalmic atropine, typically in a 1% solution or ointment, is used for eye exams and conditions like uveitis. Oral atropine, often combined with diphenoxylate (e.g., Lomotil), treats diarrhea, with 0.025 mg atropine per tablet. Nasal formulations have been studied for severe rhinorrhea, using concentrations like 0.050% and 0.075%. Dosage is always determined by a healthcare provider.

DRUG INTERACTIONS

Atropine can interact with several medications, potentially enhancing side effects or reducing therapeutic effectiveness. When taken with other anticholinergic drugs such as antihistamines, tricyclic antidepressants, or certain antipsychotics, atropine’s anticholinergic effects—like dry mouth, blurred vision, constipation, and urinary retention—can be intensified. It may also counteract the effects of cholinesterase inhibitors used in Alzheimer’s disease, reducing their efficacy. Combining atropine with monoamine oxidase inhibitors (MAOIs) or narcotic analgesics can increase the risk of central nervous system disturbances or gastrointestinal complications, such as paralytic ileus.

FOOD INTERACTIONS

Atropine has few direct interactions with food, but certain dietary factors can influence its absorption and side effects. While high-fat meals may slightly delay the absorption of oral atropine, this effect is generally not clinically significant. Alcohol should be avoided during atropine use, as it can amplify side effects such as dizziness, drowsiness, and blurred vision due to its impact on the central nervous system. Additionally, foods and beverages that contribute to dry mouth—like caffeine or spicy foods—can worsen atropine-induced dryness and discomfort.

CONTRAINDICATIONS

Atropine is contraindicated in conditions that may be aggravated by its anticholinergic effects, including closed-angle glaucoma, obstructive gastrointestinal disorders, and tachycardia. As an anticholinergic drug, atropine works by blocking the action of acetylcholine, a neurotransmitter responsible for regulating various involuntary bodily functions.

SIDE EFFECTS

• Enlarged, dilated, or bigger pupils

• Blindness

• Blurred vision

• Chest pain, discomfort, or tightness

• Cough

• Decreased vision

• Reduced urine volume

• Decreased frequency of urination

• Difficulty passing urine

• Difficulty swallowing

• Dizziness

• Eye pain

OVER DOSE

An overdose of atropine can cause severe anticholinergic symptoms such as rapid heartbeat, extreme dryness of the mouth, flushed skin, blurred vision, confusion, hallucinations, and difficulty urinating. In severe cases, it may lead to seizures, respiratory failure, or coma. Immediate medical attention is required in cases of suspected atropine overdose, and treatment is primarily supportive, including the possible use of physostigmine as an antidote under medical supervision.

TOXICITY

Atropine toxicity, also referred to as anticholinergic toxidrome, occurs when an overdose excessively blocks the action of acetylcholine at muscarinic receptors in the body. This disruption causes severe symptoms that can affect the central nervous system, cardiovascular system, eyes, and skin. In children, even small doses—around 10 mg or less—can be potentially fatal.