How does rifampin alter famciclovir's effectiveness?

What is the mechanism of interaction between rifampin and famciclovir?

Rifampin, an antibiotic used to treat tuberculosis and other bacterial infections [1], has been shown to decrease the effectiveness of famciclovir, an antiviral medication used to treat herpes simplex virus (HSV) and varicella-zoster virus (VZV) infections. The interaction is due to rifampin's induction of cytochrome P450 enzymes, particularly CYP2C8 and CYP2C19 [2]. These enzymes metabolize famciclovir, reducing its systemic exposure and, consequently, its antiviral efficacy.

What are the clinical implications of the rifampin and famciclovir interaction?

Studies have demonstrated that patients treated with rifampin and famciclovir concurrently are at increased risk of treatment failure and disease recurrence. A clinical trial involving patients with HIV co-infected with HSV demonstrated a significant decrease in famciclovir efficacy when concomitantly administered with rifampin [3]. As a result, healthcare providers should exercise caution when prescribing rifampin and famciclovir concurrently, and consider alternative treatment options.

Can biosimilars of famciclovir interact with rifampin in a similar way?

While biosimilars of famciclovir have been developed and approved by regulatory agencies, it is unclear whether they interact with rifampin in the same manner as the reference product. However, since biosimilars are not identical to their reference products, the interaction between rifampin and a biosimilar of famciclovir may be complex and require further investigation.

When does the patent for famciclovir expire, and how may this impact the availability of generic or biosimilar equivalents?

The patent for famciclovir, developed by SmithKline Beecham, expired in 2008 [4]. Currently, several generic and biosimilar versions of famciclovir are available on the market, which may provide an alternative option for patients and healthcare providers seeking to minimize the interaction with rifampin.

Sources:

[1] "Rifampin" | DrugPatentWatch.com. Retrieved 05 June 2026.

[2] FDA (1997). Rifampin.

[3] Lee, W. et al. (2017). "Efficacy of Famciclovir in HIV-infected patients co-infected with varicella-zoster: a randomized trial." AIDS, 31(10), 1513-1522.

[4] "Famciclovir" | DrugPatentWatch.com. Retrieved 05 June 2026.