We analyzed 198 illicit heroin samples from Andalusia (southern Spain) to determine the contents of various metals (cadmium, calcium, copper, iron, manganese and zinc) with a view to investigating a new aspect of the drug purity and the conditions under which the drugs are used by addicts. Calcium was found in 93.4% of the samples and always at high concentrations, which can be ascribed to adulteration of the heroin by addicts with thinners and excipients containing salts of this metal such as calcium bicarbonate. Also, all samples were found to contain variable amounts of zinc and substantial amounts of iron, probably because it is the most common metal found in metal containers used in the extraction of morphine from the opium poppy. Only cadmium and, to a lesser extent, zinc, copper, and iron, are among the metals detected in heroin that can increase the inherent toxicity of the drug while always taking into account the maximum values.