Detrimental effects

Weeds are plants which grow where people do not need them. People have spread plants above their native ranges and some of these introduced plants become invasive, damaging existing ecosystems through displacing native species. An Invasive plants cause billions of dollars in crop losses yearly through displacing crop plants they increase the cost of production and the use of chemical means to control them affects the environment.

Plants may cause harm to animals, involving people. Plants which produce windblown pollen invoke allergic reactions in people who suffer from hay fever. A broad variety of plants are harmful. Toxalbumins are plant poisons fatal to most mammals and act as a serious deterrent to consumption. Various plants cause skin irritations when touched, like as poison ivy. Certain plants contain psychotropic chemicals that are ingested and extracted or smoked, involve tobacco, cannabis (marijuana), opium and cocaine. Smoking causes death or even damage to health, although some drugs may also be harmful or fatal to people. Both illegal and legal drugs derived from plants may have negative effects on law enforcement costs and the economy affecting worker productivity. Many plants cause allergic reactions when ingested although other plants cause food intolerances which negatively affect health.