Recently, use of medicinal plants in Mexico and around the world has increased. However, in order to guarantee safe use of medicinal plants, it is necessary to quantify their content of heavy metals. In Mexico there is little information regarding content of toxic elements in medicinal plants, and these are consumed directly without a prior analysis of heavy metals. In this study, concentration of two toxic elements: cadmiom and lead (Cd and Pb) and a micronutrient (iron [Fe]) was quantified in three medicinal plants (Justicia spicigera, Arnica montana, and Hamelia pantens) collected from three different locations in two municipalities in the Huasteca Potosina, Mexico. Cd and Pb are two toxic elements with no biological function, and can cause severe damage when introduced into food chain, whereas iron is an essential element. However, iron is a major element that can form oxides and entrap trace elements such as Pb and Cd, and induce bioaccumulation of these elements in plants. Also, in the Huasteca Potosina, the three medicinal plants of this study are used for empirical treatment of iron deficiency. Therefore, it was important to study concentration of the three elements in these medicinal plants. Results of this analysis showed that 50% and 22% of herbal preparations contain higher contents of Cd and Pb than those considered as safe, respectively. In addition, the three medicinal plants could be an important source of Fe. Results suggest that collection of medicinal plants should be carried out in sites free of potentially dangerous toxic elements.