There are two main classes of medicinal preparations — herbal and pharmaceutical. Pharmaceutical drugs are either synthesized or refined substances. On the other hand, herbal medicine is defined by the World Health Organization (WHO) as labelled and final medicinal products containing parts of plants (underground or aerial), plant materials, or combinations of both, as their main active ingredient. Plant materials may come in the form of gums, juices, essential oils, fatty oils and other similar substances. Herbal drugs may also contain starch or other additives to improve the needed adhesive quality for preparing tablets or pills.