Coca



Coca refers to the two species family of Erythroxylaceae (Erythroxylum coca and E. novogranatense), native to western South America, and is best-known as the source of cocaine. Unlike some psychoactive plants, the US recognizes it has some benefit, and so allows a few people companies to import it to produce medicine and soft drinks. Unlike other psychoactive plants, it is illegal to keep in your house. The plant is considered illegal in most other countries as well, with the exceptions being those immediately around the Andes where it's been grown and chewed or drunk as tea for millennia, much to the chagrin of the US and the international community who classed it as Schedule 1 in the 1961 Single Convention on Narcotic Drugs. Since the mid-2000s, those few countries have been actively fighting the stigma associated with coca due to cocaine. They're probably annoyed that other countries have taken to spraying herbicides on their land.

History
Coca has been grown and consumed in the Andes for thousands of years, where its use is least as old as agriculture. Its popularity slowly spread throughout the region until the 19th century, when Europeans discovered how to extract pure cocaine from the leaves. Then its popularity exploded around the world, until people began to realize that cocaine wasn't all that great after all, and in their zeal to undo the damage, the world made the mere existence of the plant mostly illegal. Nice one, guys.

Scientific research
Unlike other drugs, there's not all that much research into coca itself. What little there is has mostly been done via overarching societal and personality studies covering entire populations, cultures, and the like.

1950 Report on the Commission of Inquiry on the Coca Leaf
In the late 1940s, Peru asked the UN to study coca in their country, specifically the traditional uses of chewing and brewing tea. Bolivia subsequently requested that the study also include their country, and the resulting report was the first-ever wide-scale study on coca leaves.

The report contains and, to its credit, refutes racist language, including (but certainly not limited to): Despite such prejudices, the report compiled a huge amount of data and generally comes across as not hostile to coca (classifying its use as a habit, not an addiction &mdash; pages 31 and 93) while discouraging its use due in part due to "induc[ing] … undesirable changes of an intellectual and moral character."
 * "A doctor of Cuzco … regarded [the Indian] as racially 'oligophrenic' and 'lazy'" &mdash; Page 28, section 'Psychical alterations'
 * "The mental decay and the social inferiority of the Indian are due to a) a lack of education; and b) the influence of two toxic substances, i.e. coca leaf and alcohol" &mdash; A Cochabamba physician quoted on page 29, section 'Psychical alterations'
 * "The usual opinion of the fighters against coca-leaf chewing is that it has led to a general degeneration of the Indian race. … This race, it is said, once produced a high culture and now lives at a primitive level." &mdash; Page 29, section 'Race degeneration'

Considering this was the study at the time of socio-political effects of coca, it's likely it had an impact on the 1961 decision to classify the leaf, not just cocaine, under Schedule 1.

1995 Cocaine Project
Not to be outdone by the people from the 1950s, the World Health Organization performed a study (funded by the UNICRI (United Nations Interregional Crime and Justice Research Institute)) using the work of some 40 researchers. It was, according to itself, the largest global study on cocaine ever. This study included coca leaves, although unlike the 1950 report, wasn't focused on coca, but rather included it as kind of an afterthought.

The report was controversial only in that it suggested coca wasn't the actual spawn of Satan, and that tobacco and alcohol caused far more societal problems than either coca or cocaine. However, the US, who was is eyebrow-deep in the war on drugs, couldn't allow that, and so threatened to pull funding from the WHO unless it produced results which agreed with its current anti-coca policies. As such it was never published.

Physiological effects
Unfortunately there haven't been all that many studies on coca leaves. However, what studies there have been have shown fairly benign, even beneficial, symptoms. For the ease of reading, have a bullet-list:
 * Suppresses hunger and thirst
 * Relieves oral and gastrointestinal pain
 * Mild stimulant similar to coffee
 * Does not form addiction or dependance (unlike coffee)