What is the best antibiotic for a patient with allergies to sulfonamides (sulfa) and erythromycin (Erythromycin) for the treatment of onychomycosis (toenail infection)?

From the Guidelines

For a patient with allergies to sulfa and erythromycin who has a toenail infection, fluconazole is a suitable antibiotic choice, given its efficacy and safety profile as an alternative to terbinafine or itraconazole. The recommended oral regimen for fluconazole is 450 mg once weekly for at least 6 months for toenail infections, as this dosing regimen has been associated with higher cure rates 1. Fluconazole has some activity against dermatophytes and some Candida species, which are common causes of onychomycosis. Although it is currently not licensed for use in onychomycosis, fluconazole offers the advantage of improving compliance and reducing treatment costs due to its once-weekly dosing regimen.

Key points to consider when prescribing fluconazole include:

• Its long half-life, allowing once-daily dosing, although the recommended regimen for onychomycosis is once-weekly 1
• The need to adjust the dose depending on creatinine clearance, as it is excreted predominantly in the urine 1
• Common adverse effects, such as headache, skin rash, gastrointestinal complaints, and insomnia, which may lead to treatment discontinuation in some patients 1
• Its weaker inhibition of the cytochrome P450 enzymes compared to itraconazole, resulting in fewer drug interactions 1

It's essential to note that toenail infections are typically fungal rather than bacterial, so traditional antibiotics are not effective. Treatment requires antifungal medications, and complete resolution may take months as the infected nail grows out. Monitoring liver function tests during treatment and advising patients on the expected cure rates are also crucial aspects of management. Given the patient's allergies to sulfa and erythromycin, fluconazole emerges as a viable option, with its once-weekly dosing regimen potentially improving compliance compared to daily terbinafine or itraconazole 1.

From the FDA Drug Label

Terbinafine tablets, USP are indicated for the treatment of onychomycosis of the toenail or fingernail due to dermatophytes (tinea unguium). The best antibiotic for the patient with a toenail infection and allergies to sulfa and erythromycin is not an antibiotic, but an antifungal: terbinafine.

• Key points:
  • Terbinafine is an allylamine antifungal that inhibits biosynthesis of ergosterol.
  • It has been shown to be active against most strains of dermatophytes, including Trichophyton mentagrophytes and Trichophyton rubrum.
  • The efficacy of terbinafine in the treatment of onychomycosis is supported by clinical trials, with mycological cure rates of 70% in patients with toenail infections 2, 2, 2.
  • Important note: The patient's allergies to sulfa and erythromycin do not affect the use of terbinafine, as it is not a sulfonamide or a macrolide antibiotic.

From the Research

Treatment Options for Toenail Infection

The patient has allergies to sulfa and erythromycin, which limits the treatment options. Considering the toenail infection, the following points are relevant:

• Onychomycosis is a common fungal infection of the nail, and treatment options include systemic and topical therapies 3.
• Systemic antifungals, such as terbinafine, itraconazole, and fluconazole, are effective in treating onychomycosis, but may have risks of systemic toxicity and drug-drug interactions 4, 5, 6.
• Terbinafine is considered the treatment of choice for onychomycosis, especially when a dermatophyte is the pathogen 4, 3.
• Topical antifungals, such as efinaconazole and tavaborole, are also available and may be considered for mild to moderate onychomycosis or when oral antifungal agents are contraindicated or cannot be tolerated 5, 6.

Safety Considerations

When choosing a treatment option, safety is a crucial consideration:

• The risk of terbinafine-induced hepatotoxicity is negligible in healthy patients 6.
• Systemic therapies, especially azole antifungals, are associated with numerous drug-drug interactions, some of which are life-threatening and fatal 6.
• Topical antifungals are well tolerated and generally safe, with only potential local side effects 6.

Recommended Treatment

Based on the available evidence, the following treatment options may be considered:

• Oral terbinafine, as it is considered the treatment of choice for onychomycosis, especially when a dermatophyte is the pathogen 4, 3.
• Topical antifungals, such as efinaconazole and tavaborole, may be considered for mild to moderate onychomycosis or when oral antifungal agents are contraindicated or cannot be tolerated 5, 6.