Which antibiotics are safe to use in a patient with myasthenia gravis?

Antibiotics Safe in Myasthenia Gravis

Antibiotics to Absolutely Avoid

Fluoroquinolones (levofloxacin, moxifloxacin, ciprofloxacin) and aminoglycosides (amikacin, gentamicin, tobramycin) are contraindicated in myasthenia gravis and must be avoided due to their neuromuscular blocking activity that can precipitate respiratory failure. 1, 2, 3, 4

• Fluoroquinolones carry an FDA boxed warning specifically stating they may exacerbate muscle weakness in myasthenia gravis patients, with postmarketing reports of deaths and requirement for ventilatory support 3, 4
• These agents block neuromuscular transmission by decreasing miniature endplate potential amplitudes in a dose-dependent manner 5
• Aminoglycosides (amikacin, gentamicin, tobramycin, streptomycin) are explicitly contraindicated as they impair neuromuscular transmission 6
• Macrolide antibiotics (azithromycin, erythromycin, clarithromycin) should be avoided as they can worsen myasthenic symptoms 1, 2

Generally Safe Antibiotic Classes

Beta-lactams (penicillins and cephalosporins) are considered the safest antibiotic class for myasthenia gravis patients, though rare exacerbations have been reported with amoxicillin requiring close monitoring.

Penicillins and Cephalosporins

• Cephalexin 500 mg four times daily orally is safe and commonly used for skin and soft tissue infections 6
• Cefazolin 1 g every 8 hours IV is safe for parenteral therapy 6
• Nafcillin or oxacillin 1-2 g every 4 hours IV are safe options for MSSA infections 6
• Dicloxacillin 500 mg four times daily orally is safe for oral MSSA coverage 6
• Amoxicillin/amoxicillin-clavulanate are generally safe but require close monitoring, as six documented cases showed acute MG worsening within days of administration, with all patients requiring therapeutic intervention and 1-2 months for full recovery 7

Other Safe Options

• Clindamycin 300-600 mg every 6-8 hours (oral or IV) is safe and effective for both MSSA and MRSA infections 6
• Vancomycin 30 mg/kg/day in 2 divided doses IV is the parenteral drug of choice for MRSA and safe in myasthenia gravis 6
• Linezolid 600 mg every 12 hours (IV or oral) is safe for MRSA coverage 6
• Daptomycin 4 mg/kg every 24 hours IV is safe for MRSA infections 6
• Ceftaroline 600 mg twice daily IV is safe and provides MRSA coverage 6
• Trimethoprim-sulfamethoxazole 1-2 double-strength tablets twice daily orally is safe, though efficacy is poorly documented 6
• Doxycycline/minocycline 100 mg twice daily orally are safe options, though bacteriostatic with limited recent clinical experience 6
• Tigecycline has been successfully used in a documented case of community-acquired pneumonia in an MG patient who previously experienced crisis with moxifloxacin 8

Clinical Monitoring Protocol

When prescribing any antibiotic to a myasthenia gravis patient, monitor daily for worsening muscle weakness, bulbar symptoms (speech/swallowing changes), diplopia, and respiratory compromise.

• Assess baseline respiratory function with negative inspiratory force (NIF) and vital capacity (VC) before starting antibiotics in patients with generalized MG 1, 2
• Apply the "20/30/40 rule" to identify respiratory failure risk: vital capacity <20 mL/kg, maximum inspiratory pressure <30 cmH₂O, or maximum expiratory pressure <40 cmH₂O 1
• Monitor for symptom progression within the first few days of antibiotic initiation, as most exacerbations occur within this timeframe 7
• If worsening occurs, immediately discontinue the antibiotic, switch to a safer alternative, and consider increasing immunosuppressive therapy or adding IVIG/plasmapheresis for severe cases 1, 2

Common Pitfalls to Avoid

• Do not assume all beta-lactams are completely safe: while generally the safest class, amoxicillin has documented cases of MG exacerbation requiring close monitoring 7
• Do not use fluoroquinolones even for resistant infections: the risk of precipitating myasthenic crisis outweighs any antimicrobial benefit, and alternative agents should always be found 3, 4, 8
• Do not overlook drug interactions: avoid combining antibiotics with other neuromuscular blocking agents or medications that worsen MG (beta-blockers, IV magnesium) 1, 2
• Do not delay switching antibiotics: if any worsening of MG symptoms occurs after antibiotic initiation, immediately change to a safer alternative rather than waiting to see if symptoms stabilize 8, 7