What is Mestinon (Pyridostigmine)?
Mestinon (pyridostigmine bromide) is an orally active cholinesterase inhibitor that prevents the breakdown of acetylcholine, thereby enhancing nerve impulse transmission across the neuromuscular junction. 1
Mechanism of Action
Pyridostigmine inhibits the enzyme acetylcholinesterase, which normally destroys acetylcholine at nerve endings. By blocking this destruction, the drug allows acetylcholine to accumulate and permits more effective transmission of nerve impulses across the neuromuscular junction 1. Compared to neostigmine (Prostigmin), pyridostigmine has a longer duration of action and causes fewer gastrointestinal side effects 1.
Available Formulations
Mestinon comes in three forms 1:
- Oral solution: 60 mg per teaspoonful (contains 5% alcohol)
- Standard tablets: 60 mg immediate-release
- Timespan tablets: 180 mg extended-release
Primary Clinical Uses
Myasthenia Gravis (Primary Indication)
Pyridostigmine is the first-line symptomatic treatment for myasthenia gravis, with proven efficacy in reducing muscle weakness. A 2026 randomized controlled trial demonstrated significant improvement across all outcome measures, with a mean difference of 5.3 points on the Myasthenia Gravis Impairment Index (p = 0.004) 2. This study also showed substantial cost savings (€6,565 annually) and improved quality-adjusted life years (0.106 QALYs) 2.
Neurogenic Orthostatic Hypotension (Secondary Indication)
Pyridostigmine may be beneficial in patients with syncope due to neurogenic orthostatic hypotension who are refractory to other treatments 3. The drug improves orthostatic tolerance by increasing peripheral vascular resistance and blood pressure in patients with autonomic failure 3. A 2025 meta-analysis found that pyridostigmine combined with midodrine demonstrated significant improvement in systolic orthostatic drop, though pyridostigmine alone showed non-significant benefits 4. Importantly, pyridostigmine does not worsen supine hypertension, making it particularly useful in patients where this is a concern 3, 4.
Common Side Effects
The most frequently reported adverse effects include 3, 5:
- Gastrointestinal: Nausea, vomiting, abdominal cramping, diarrhea, flatulence (91% of users report some side effects)
- Urinary: Urinary urgency and incontinence
- Muscular: Muscle cramps and twitching
- Other: Sweating, salivation, blurred vision, hyperhidrosis
In a cross-sectional study of 410 myasthenia gravis patients, 26% discontinued pyridostigmine specifically due to side effects, with diarrhea, abdominal cramps, and muscle twitching being the most common reasons 5.
Important Caveats and Pitfalls
Blood-Brain Barrier Penetration Under Stress
Pyridostigmine is a quaternary ammonium compound that normally does not cross the blood-brain barrier. However, under stress conditions, the blood-brain barrier can become permeable, allowing pyridostigmine to enter the brain and cause central nervous system effects 6. This was documented during the Persian Gulf War, where stress-related barrier disruption led to a threefold increase in CNS symptoms 6.
Platelet Aggregation Effects
Pyridostigmine can inhibit platelet aggregation in response to ADP and collagen, potentially causing bleeding tendencies in susceptible patients 7. Healthcare providers should be aware of this uncommon but clinically significant side effect, particularly if patients develop unexplained bruising or bleeding 7.
Not Effective for ALS
Despite theoretical mechanisms at the neuromuscular junction, pyridostigmine should not be used for slowing ALS progression, as it lacks mechanistic plausibility and clinical trial evidence of efficacy 8.