Ritonavir significantly increases exposure to colchicine (gout)
The pharmacokinetic interaction between ritonavir (100 mg twice daily) and colchicine (0.6 mg single dose) was investigated in 24 HIV negative subjects.
Ritonavir increased colchicine Cmax and AUC by 170% and 240% compared to colchicine alone. There was no apparent increase in the incidence of adverse events during coadministration.
Colchicine doses should be adjusted when administered with ritonavir to avoid the risk of colchicine‐related toxicities.
Colchicine is a herbal medicine used to treat gout and has a wide range of potential interactions including with antibiotics, antifungals, statins and other HIV protease inhibitors. 
Source: hiv-druginteractions.org (31 July 2012).
- Wason S et al. Clinically significant drug interaction between colchicine and ritonavir in healthy adults. 14th International Workshop on Comorbidities and Adverse Drug Reactions in HIV, Washington, July 2012. Abstract P22.
- British National formulary