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Pharmacology · Cholinesterase inhibitors

Pyridostigmine !

The standard oral medicine for the weakness of myasthenia gravis. It lasts longer than neostigmine and causes fewer gut side effects.

Updated 2026-11-01

No crystal structure of pyridostigmine bound to acetylcholinesterase exists. The figure shows edrophonium, a close chemical relative with the same charged trimethylammonium head, in the active site of Torpedo (electric ray) acetylcholinesterase, which shares the human enzyme's gorge and catalytic serine.

Target in 3D: Acetylcholinesterase. PDB structure 1AX9 (RCSB PDB, public domain). View the structure at RCSB PDB

Mechanism of action

In myasthenia gravis, antibodies reduce the number of working acetylcholine receptors at the muscle end plate. Pyridostigmine carbamylates acetylcholinesterase, so each nerve impulse leaves more acetylcholine in the synapse for longer and reaches the remaining receptors. It treats the symptom (weakness), not the autoimmune cause. It is a quaternary amine and does not enter the brain.

  1. Carbamylates acetylcholinesterase at the neuromuscular junction
  2. Acetylcholine lasts longer in the synapse
  3. More of the remaining receptors are activated
  4. Muscle strength and endurance improve

Pharmacokinetics

Absorption
Poorly absorbed by mouth, so oral doses are much larger than injected ones; regular and extended-release tablets and a liquid exist
Distribution
Does not cross the blood-brain barrier
Elimination
Mainly unchanged in the urine; lower doses may be needed in kidney disease
Onset and duration
Acts longer than neostigmine

Indications

  • Myasthenia gravis (symptomatic treatment) Myasthenia gravis →
  • Pretreatment of military personnel against certain nerve agents
  • Reversal of non-depolarizing muscle relaxants (injection)

Adverse effects

Common

  • Nausea, vomiting, diarrhoea and abdominal cramps
  • More saliva and bronchial secretions
  • Sweating and small pupils
  • Muscle cramps and fasciculations

Contraindications

  • Mechanical obstruction of the bowel or urinary tract
  • Particular caution in bronchial asthma

Key interactions

Atropine and other antimuscarinics
Relieve gut side effects but can hide the early signs of overdose and let a cholinergic crisis develop
Succinylcholine
Prolonged neuromuscular block (class effect)
Aminoglycoside antibiotics
Can add neuromuscular block; people with myasthenia gravis are particularly susceptible

What is monitored

  • Muscle strength, swallowing, speech and eyelid drooping through the day
  • Breathing: vital capacity in a crisis
  • Kidney function

Administration and safety

  • Taken at set times; the timing is matched to when strength is most needed
  • Extended-release tablets are not crushed
  • Tablets absorb moisture: kept dry in their original container

Nursing care

  • Give doses on time; a late dose can bring back weakness
  • Check swallowing before giving food or other oral medicines
  • Teach the signs of both crises: worsening weakness, trouble breathing or swallowing, heavy sweating or salivation
  • Keep atropine available

Pregnancy and breastfeeding

Safety in pregnancy and breastfeeding has not been established; benefits and risks are weighed, since uncontrolled myasthenia is itself dangerous.

Test yourself

Think of the answer, then tap the question to see it.

How does pyridostigmine improve weakness in myasthenia gravis?

By inhibiting acetylcholinesterase, so acetylcholine lasts longer and activates more of the receptors that the antibodies have spared.

Why can routine atropine with pyridostigmine be risky?

It masks the muscarinic warning signs of overdose, so a cholinergic crisis can develop unnoticed.

Which organ eliminates pyridostigmine, and what follows from that?

The kidney, mostly unchanged; people with kidney disease may need lower doses.

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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: DailyMed - PYRIDOSTIGMINE BROMIDE tablet (DailyMed (U.S. National Library of Medicine), 2026); Pyridostigmine: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2026); Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed., ch. 12 (McGraw Hill, 2023); Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed., ch. 59 (McGraw Hill, 2023).

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Representative model, for educational purposes only.