A clash of interests looms in Bolivia, where the US is keen to see cocaine production stamped out but where the new president is a former coca farmer, the BBC's Becky Branford reports.
Chewing on dark-green coca leaves is a stock tourist experience for visitors to Bolivia.
The bitter leaves - which make the mouth tingle and leave it slightly numb - increase alertness, rather like a cup of coffee. And for thousands of years coca has played a central role in Andean culture.
Indigenous Andeans have used coca leaves for thousands of years
But coca - of which Bolivia is the world's third producer - also happens to be the raw ingredient of cocaine. This has made coca cultivation in Bolivia a prominent US foreign policy concern in the region.
US-driven anti-drugs efforts in Bolivia have not always been appreciated in the regions where coca has been grown for centuries.
The recent election of the leader of the coca-growers' union, Evo Morales, as Bolivia's president has highlighted the centrality of coca to domestic politics.
Mr Morales has pledged to loosen restrictions on coca production - raising the spectre of a potential clash with the US over the issue.
Coca's cultural importance in Bolivia is "absolutely incalculable; absolutely fundamental", says Tristan Platt, an anthropologist at the University of St Andrews who has studied the history of coca cultivation in the Andean countries.
"It can be used for divination, for oracle-telling, as an offering to divinities in sacred places - it's used in rituals of all sorts. Three perfect leaves of coca are a perfect offering to a sacred entity."
This Inca figurine wears a necklace of coca leaves (image courtesy Xanthos)
Coca has been grown in the region, Mr Platt says, since at least before Christ. It was central to the Aymara kingdoms on the Altiplano (the Andean plateau) and the Incas also enjoyed it.
Although Spanish conquistadores disapproved of the local dependence on coca, says Mr Platt, they also reaped the rewards of a lucrative regional trade in the plant.
Coca also plays a practical role. It is said to alleviate symptoms of altitude sickness in this elevated region. It also suppresses hunger and provides energy - a boon to poor families and labourers, including miners and agricultural workers, in impoverished Bolivia and Peru.
Most medical opinion holds that the leaf, which is thought to be a source of calcium, iron, and vitamins A and C, is neither harmful nor addictive.
War on drugs
But Bolivia's emergence as a primary producer of coca for cocaine in the 1980s and 1990s triggered heightened US involvement in the country - an involvement which, ironically, lies in part behind the rise of President Morales.
Since the 1990s, Washington and European countries have injected an estimated $700m (£400m) of development aid into the Chapare, a primary coca-growing region along with the Yungas.
The money was intended to be used to develop alternative agricultural industries to coca - such as bananas, coffee, and palm hearts.
But these programmes are largely judged to have failed. In many cases, rural communities in the remote region found they were unable to make profits on their harvests once expensive transport costs had been paid.
The flipside of this policy was a tough coca eradication campaign, and the result, in many cases, was increasing poverty among farmers both unable to make a living on alternative agricultural products and prohibited from farming coca. In addition, US drug-enforcement and military experts in the region were commonly identified as complicit in brutal government campaigns against the coca-growers, or cocaleros.
In Chapare, resistance against these policies was led by a union organiser - Evo Morales.
Mr Morales was an eloquent exponent of resentment against the US-backed eradication policies, and also articulated wider anger about so-called "neo-liberal" measures in Bolivia, also perceived to be US-driven.
Coca cultivation is not completely banned in Bolivia. Since 1988, Bolivian law has allowed for 12,000 hectares (30,000 acres) of coca to be cultivated for legitimate uses, and in 2004 Mr Morales won an agreement with the government allowing for a further 3,200 hectares to be grown in his Chapare region.
But experts acknowledge that actual coca cultivation has always significantly exceeded this limit.
While coca cultivation did drop radically in Bolivia from the mid-1990s to 2002, it has since begun to rise again, registering a 17% increase from 2004 to 2005, according to the UN.
Between 2003 and 2004, reported seizures of cocaine base rose 18% to eight tons, it said - the fourth consecutive annual increase.
Mr Morales' victory in presidential elections has stoked fears in Washington that Bolivia's efforts to control cocaine production could be seriously undermined.
In a US state department briefing on Bolivia, President Morales is described as an "illegal-coca agitator", and Anne Patterson, Assistant Secretary for International Narcotics and Law Enforcement Affairs, recently gave this warning:
"Our relationship with the new Morales government... will depend upon the policies it adopts on a wide range of issues, including counternarcotics... More coca cultivation will only serve the interest of drug traffickers."
'Coca si, cocaina no'
Mr Morales, meanwhile, has suspended eradication programmes and says he would like to develop the industry in legitimate coca-based products. He has welcomed an EU study which is currently assessing the size of the legal market for coca.
He has also appointed Felipe Caceres - himself a cocalero and union leader - as the country's anti-drugs chief.
Mr Caceres explained the thinking behind the new policy, dubbed "coca si, cocaina no" (coca yes, cocaine no).
"The policy of the preceding neo-liberal governments was aimed, under the direction of the US embassy, at eradicating coca, and the policy did not concern itself with the poverty and the social and cultural aspects that were tied to cultivation.
An ad for coca-based health and beauty products (image courtesy Jacek Zoch)
"We are proposing a front-on assault on the real drugs trade... against money trading and the so-called precursor chemicals which are used to manufacture cocaine.
"The coca leaf itself is not a drug and the farmer is not a drug dealer... to this day, however, our prisons are crammed with simple and poor farmers, and not with the real drugs bosses."
The new Bolivian administration has called for a 1961 UN convention which declares coca an illegal narcotic to be scrapped, allowing it to export coca-based products which could include tea bags, soap, shampoo, biscuits, wine, and even diet pills.
But there were warm greetings between Mr Morales and US Secretary of State Condoleezza Rice when they met in March - with Mr Morales even presenting Ms Rice with a charango (a Bolivian ukulele) - decorated with lacquered coca leaves.
Morales, right, presents Rice with a coca-decorated ukulele
Mr Morales is trying to appease both the US and his cocalero supporters, says John Crabtree, author of Patterns of Protest: Politics and Social Movements in Bolivia.
"On the one hand, the cocaleros are among Morales' prime constituents... but on the other hand, he's going to come under enormous pressure from the US to do something" to curb coca production.
Mr Crabtree points out that if he does not, Mr Morales may put at risk trade preferences which Bolivia enjoys in the US market - which are very important to some Bolivian textile and handicraft producers.
These trade preferences are explicitly linked with anti-drug programmes, as the name of the US act which governs the agreement - the Andean Trade Promotion and Drug Eradication Act - suggests.
"Morales is walking a tightrope," says Mr Crabtree.