There has been an isolated report of proguanil interacting with warfarin (Armstrong et al 1991), and the anticoagulant effect of warfarin is possibly enhanced by tetracyclines including doxycycline (Joint Formulary Committee 2012). Atovaquone is a component of the antimalarial drug Malarone, and an isolated report of an interaction with warfarin was
documented by Hidalgo et al in 2011. It is therefore advisable that travellers taking warfarin have their baseline international normalised ratio (the time it takes for blood to clot) taken before starting their antimalarial tablets, and rechecked after oneweek (Chiodini et al 2007). This allows time before departure for any warfarin dosage adjustments to be made as appropriate.