Siam weed along highways, herbal medicine or poison?

The levels of five metals (Cd, Hg, Mn, Ni, and Pb) were determined in Siam weed (Chromolaena odorata) collected along Nigerian highways with a view to ascertaining the suitability or otherwise of the plant species for therapeutic purposes. Samples were acid digested and the levels of the metals in the digestates were determined using an atomic absorption spectrophotometer. The average concentrations recorded for all metals, with the exception of Cd, exceeded the maximum permissible concentrations (MPCs) of metals in natural systems for the protection of human health. It was only at the reference site that the concentrations of the five metals were within the MPC. Going by the results of this study, it is obvious that plant samples collected within the vicinity of the highways are not suitable for therapeutic purposes due to their high metals' burdens. Therefore, for therapeutic purposes, vegetative parts of medicinal plants should be sourced at very far distances from highways to safeguard consumers of herbal medicines from toxicity associated with the ingestion of these metals.