Can't say if it is routine in Mexico, but the one time I flew into Ixtapa/Zihue the Mexican Customs agents demanded to see every single pill that anyone on the charter had with them - yes, it took about an hour for them to look into every single toiletries kit, and no, that airport did not have A/C. I have no idea if they knew what any of the pills they saw were for, but they let me keep my Bayer aspirin.