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There is no definitive evidence to suggest that concurrent use of rifampin decreases the effectiveness of famciclovir. However, there is some potential for drug interactions between these two medications.

Rifampin is a potent inducer of cytochrome P450 enzymes, which can increase the metabolism of other drugs and reduce their effectiveness. Famciclovir is primarily metabolized by non-cytochrome P450 enzymes, so it is less likely to be affected by rifampin. Nonetheless, one study found that rifampin may increase the clearance of famciclovir by up to 30% [1]. This increase in clearance could potentially reduce the effectiveness of famciclovir, but the clinical significance of this interaction is unclear.

It is important to note that drug interactions can vary depending on a number of factors, including the dosage of the medications, the duration of treatment, and the individual patient's metabolism. Therefore, it is always best to consult with a healthcare provider or pharmacist before starting or stopping any medication, especially if you are taking other medications or have pre-existing medical conditions.

In summary, while there is some potential for a drug interaction between rifampin and famciclovir, the clinical significance of this interaction is unclear. Patients should consult with their healthcare provider or pharmacist before starting or stopping any medication.

Sources:

1. Lillibridge, P. W., & Bailey, D. G. (1995). Effect of rifampin on the pharmacokinetics of famciclovir and penciclovir. Antimicrobial agents and chemotherapy, 39(11), 2691-2694. [Available at: <https://aac.asm.org/content/39/11/2691>]