Since so many plants contain chemicals which affect the human organism, abuse of plant drugs may be unintentional, i.e. through the unwitting ingestion of natural toxic plant substances. Typical drug reactions may also occur as a result of ingesting chemical fungicides, pesticides or other chemical preparations applied to plants. Many of these chemicals interfere with important enzyme systems and induce neurometabolic disorder. However, some plants yield psychoactive substances with little or no processing and may be readily abused.
The history of the human race has also been a history of drug use. Since earliest times, herbs, roots, bark, leaves and plants have been used to relieve pain and help control disease. Drugs easily extracted from plants by chewing, infusion or simple extractive processes such as maceration include mescaline, khat, coca, and betel. In tropical countries millions of persons in rural areas may be addicted to these substances.