In a sequential pharmacokinetic study, healthy male volunteers (n=8) were administered a single oral dose of 1000 mg mebendazole, in absence and after short-term (2 doses) and long-term (8 days) treatment with ritonavir 200 mg twice daily. A trend towards an increase in mebendazole exposure after short-term intake of ritonavir was seen. However long-term administration resulted in a significant decrease in mebendazole AUC0-24 (57%) and Cmax (59%). Tmax and the terminal half-life were not significantly changed. The observation is most likely due to a ritonavir-mediated induction of metabolising enzymes (CYP2C9, CYP1A2, UGTs) or transporters, although changes in absorption are also possible. These findings need to be replicated with a key boosted PI such as lopinavir/ritonavir.
Effect of ritonavir on the pharmacokinetics of the benzimidazoles albendazole and mebendazole: an interaction study in healthy volunteers. Corti N, Heck A, Rentsch K, et al.Eur J Clin Pharmacol. 2009;65:999-1006.

In 8 patients, the [14C]-aminopyrine breath test (ABT) and maximum serum concentration of mebendazole following a dose of 1.5 g of mebendazole three times daily were determined before and after treatment with cimetidine (400mg three times daily for 30 days). Serum mebendazole concentrations were measured in blood samples taken 2 h after each drug intake. Cimetidine lowered the 14CO2 specific activity (SA) at 1 h and increased the maximum serum concentration of mebendazole (82.3ng/ml vs 55.7ng/ml).
Cimetidine increases serum mebendazole concentrations. Implications for treatment of hepatic hydatid cysts. Bekhti A, Pirotte J. Br J Clin Pharmacol. 1987;24:390-2.