Amoxicillin and Myasthenia Gravis
Amoxicillin can be used in patients with myasthenia gravis, but requires close monitoring as it has been documented to cause acute exacerbations in some patients, despite penicillins traditionally being considered safer than other antibiotic classes like fluoroquinolones, aminoglycosides, and macrolides. 1, 2
Risk Assessment
While current guidelines specifically warn against fluoroquinolones, aminoglycosides, and macrolide antibiotics in MG patients 1, 3, amoxicillin is notably absent from these high-risk lists. However, clinical evidence demonstrates a real risk:
- A case series documented six MG patients who developed acute worsening of symptoms after amoxicillin or amoxicillin/clavulanate treatment, with symptoms starting within a few days of antibiotic administration 2
- All six patients experienced worsening of their MGFA clinical classification and required therapeutic intervention with dosage increases or new medications 2
- Historical data from 1986 showed ampicillin (a closely related penicillin) aggravated symptoms in myasthenic patients and increased electrical decrements in experimental autoimmune MG 4
Clinical Decision-Making Algorithm
If amoxicillin is chosen for infection treatment in an MG patient:
Baseline assessment: Measure negative inspiratory force (NIF) and vital capacity (VC) before starting treatment, applying the "20/30/40 rule" (VC <20 ml/kg, maximum inspiratory pressure <30 cmH₂O, or maximum expiratory pressure <40 cmH₂O indicates high risk) 3
Daily monitoring: Perform daily neurological evaluation focusing on muscle strength, bulbar symptoms (speech, swallowing difficulties), diplopia, and ptosis 1, 3
Respiratory surveillance: Monitor respiratory function frequently, especially in patients with generalized MG (MGFA class III-V), as pulse oximetry and arterial blood gases may not capture early decompensation 3
When to Avoid Amoxicillin
Consider alternative antibiotics if:
- The patient has a history of previous MG exacerbation with penicillins 2, 5
- The patient has compromised baseline respiratory function (meeting the 20/30/40 rule criteria) 3
- The patient has symptomatic, generalized MG, as these patients are especially vulnerable to drug-induced exacerbations 6
Management of Exacerbation
If the patient deteriorates on amoxicillin:
- Immediately discontinue the antibiotic 2
- Hospitalize for ICU-level monitoring if respiratory compromise develops 3
- Administer high-dose corticosteroids: methylprednisolone 1-2 mg/kg/day IV 3
- Initiate rapid immunomodulation with either plasmapheresis (5 sessions over 5 days) OR IVIG (2 g/kg total dose over 5 days) 3
- Most patients achieve full recovery to baseline within 1-2 months after discontinuation 2
Critical Pitfall
The major clinical trap is assuming all penicillins are completely safe in MG simply because they are not listed among the highest-risk antibiotics. 2, 4 While amoxicillin carries lower risk than fluoroquinolones or aminoglycosides, it is not risk-free and requires vigilant monitoring, particularly in patients with generalized or symptomatic disease 6.