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Leg ulcer - venous - Management
What issues should I consider before prescribing erythromycin or clarithromycin?

  • Consider using clarithromycin in people who have have a history of gastrointestinal effects when taking erythromycin. Clarithromycin is usually associated with milder gastrointestinal adverse effects [BNF 54, 2007].
  • Possible enhanced effect of aminophylline or theophylline, and carbamazepine (due to cytochrome P450 enzyme inhibition). Consider using clarithromycin (or azithromycin) in preference to erythromycin as it causes only modest increases in levels [Baxter, 2006]. Check levels if toxicity is suspected (e.g. palpitations, nausea, headache), or 48 hours after starting concurrent treatment with erythromycin.
  • Possible enhanced effect of carbamazepine (due to cytochrome P450 enzyme inhibition). Consider using azithromycin instead (no interaction reported). Alternatively, consider reducing the dose of carbamazepine by 30–50% during treatment with erythromycin or clarithromycin, and advise people to report symptoms of toxicity (e.g. dizziness, diplopia, ataxia, confusion) [Baxter, 2006].
  • Possible enhanced effect of warfarin. Reduce warfarin dose by 50% (especially in people over 60 years of age, who clear warfarin slowly) and monitor weekly for up to 3 weeks after stopping.
  • Possible QT-interval prolongation. If possible, avoid giving erythromycin or clarithromycin if the person is already taking a drug that can potentially prolong the QT interval (e.g. anti-arrhythmics, antipsychotics, tricyclic antidepressants) or if the person has hypokalaemia which also increases the risk of QT prolongation [Baxter, 2006].
  • Increased risk of myopathy in people taking atorvastatin or simvastatin (due to cytochrome P450 enzyme CYP3A4 inhibition) [CSM, 2004]. Stop atorvastatin or simvastatin for the duration of erythromycin or clarithromycin treatment [ABPI Medicines Compendium, 2009], or advise people to report muscle symptoms if atorvastatin or simvastatin need to be continued [Baxter, 2006]. Alternatively, consider using azithromycin instead (as it is reported to have little effect on cytochrome P450 enzymes) but advise people to report muscle symptoms [Sweetman, 2005].

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