Can topical Metrogel (metronidazole) cause myositis?

Topical Metrogel (Metronidazole) and Myositis

Topical metronidazole (Metrogel) does not cause myositis based on current medical evidence and guidelines.

Understanding Myositis

Myositis is an inflammatory condition of skeletal muscles characterized by:

• Muscle weakness (primarily in proximal extremities)
• Difficulties in standing up, lifting arms, and moving around
• Sometimes accompanied by myalgia (muscle pain)
• Elevated creatine kinase (CK) levels

Known Causes of Myositis

Myositis can be caused by various factors:

1. Immune checkpoint inhibitors (ICIs) - particularly anti-PD-1/PD-L1 agents 1
2. Certain medications - including statins and methimazole 2
3. Infectious agents - bacterial, viral (influenza, HIV, SARS-CoV2) 3
4. Autoimmune conditions - idiopathic inflammatory myopathies 1
5. Malignancies - paraneoplastic syndromes 3

Topical Metronidazole Safety Profile

Topical metronidazole has been extensively studied and has shown:

• Minimal systemic absorption - Only trace amounts enter the bloodstream 4
• Low adverse event profile - Primarily limited to local reactions 5
• No documented cases of myositis - Unlike oral or IV metronidazole which can have systemic effects 6

Safety studies specifically on metronidazole 1% gel have demonstrated:

• Not irritating under occlusive application
• Low potential for sensitization reactions
• No evidence of phototoxic or photoallergic reactions 5

Common Side Effects of Topical Metronidazole

The most frequently reported adverse effects are limited to local reactions:

• Stinging (≤2%)
• Dryness (≤2%)
• Burning (≤2%)
• Itching (≤2%) 4

Important Distinctions

1. Topical vs. Systemic Administration:

   • Topical metronidazole has minimal systemic absorption
   • Systemic metronidazole (oral/IV) can cause more widespread effects

2. Myalgia vs. Myositis:

   • Myalgia (muscle pain) is different from myositis (muscle inflammation)
   • Neither is documented as a side effect of topical metronidazole

Clinical Implications

For patients concerned about muscle symptoms while using Metrogel:

• Evaluate for other causes of myositis
• Consider medication review for other potential myotoxic drugs
• Assess for signs of systemic inflammatory conditions
• Measure CK levels if clinically indicated

Conclusion

Based on the available evidence, topical metronidazole (Metrogel) does not cause myositis. Any muscle inflammation occurring in a patient using topical metronidazole is more likely due to other causes and warrants further investigation.