Methylcobalamin is the only substance in biological systems that is known to be able to methylate mercury. (Note that methylcobalamin is the substance used by bacteria to perform methylation on environmental mercury, and the resulting methylmercury is concentrated in the food chain up to the large predatory fish and enters the human diet.) Methylmercury can readily cross the blood-brain barrier. Methylation of mercury by methylcobalamin has been reported in the literature to occur within the bodies of guinea pigs in laboratory experiments. Perque B12 is sublingual to compensate for poor B12 absorption in the gut of many people.