Cephalexin Does Not Trigger Myasthenia Gravis
Cephalexin is not known to trigger or exacerbate myasthenia gravis and is considered safe to use in patients with MG. 1
Evidence-Based Safety Profile
The most authoritative guideline addressing drug safety in MG patients specifically identifies medications that should be avoided or reviewed 1:
- Antibiotics to avoid in MG include: fluoroquinolones, aminoglycosides, and macrolide antibiotics 1
- Cephalosporins are notably absent from this list of contraindicated medications 1
- Beta-lactam antibiotics (penicillins and cephalosporins) work through different mechanisms than the drugs known to worsen MG 2
Mechanism of MG Exacerbation
Drugs that worsen MG typically interfere with neuromuscular transmission through several pathways 2:
- Pre- or postsynaptic ion channel effects 2
- Acetylcholinesterase interference 2
- Direct neuromuscular junction blockade (as seen with aminoglycosides and fluoroquinolones) 1, 2
Cephalexin does not operate through any of these mechanisms and maintains its antibacterial activity through beta-lactam ring-mediated bacterial cell wall synthesis inhibition 1
Clinical Context and Caveats
While cephalexin itself does not trigger MG, important considerations include:
- Penicillins (structurally related to cephalosporins) have rare case reports of MG exacerbation: Six cases of amoxicillin-associated MG worsening have been documented, with symptoms developing within days and requiring 1-2 months for full recovery 3
- However, penicillins are still generally considered safe in MG 3, 2
- Cross-reactivity between penicillins and cephalosporins is low (2-4%) and primarily relates to allergic reactions, not MG exacerbation 1
Practical Recommendations
When prescribing cephalexin to MG patients:
- Cephalexin can be used without specific MG-related precautions 1
- Standard dosing applies: 500 mg three to four times daily for skin/soft tissue infections 4, 5
- Monitor for infection-related MG worsening: Systemic illness itself can exacerbate MG symptoms independent of antibiotic choice 2
- Avoid the truly contraindicated antibiotics: fluoroquinolones, aminoglycosides (including topical formulations), and macrolides 1, 6