In Seattle this weekend, a school bus rigged up as a food truck will start selling items infused with marijuana. The menu includes truffle popcorn, peanut butter and jelly, and a Vietnamese pork banh mi, reports the Los Angeles Times.

It may become an increasingly typical sight in Washington State, one of two states where the possession and sale of pot for recreational purposes is now legal. (Colorado is the other state.) And it could spread as the trend toward legalizing marijuana continues.

On the November ballot in Oregon and Alaska are measures allowing the sale of recreational marijuana to adults. Meanwhile, Florida voters will decide on a constitutional amendment legalizing the use of marijuana for medical purposes. That would make it the 24th state, plus the District of Columbia, to legalize medical marijuana.

All of this fits public opinion polls showing greater acceptance of the trend, particularly among younger Americans. But a United Nations report offers a cautionary note.

Use of marijuana around the world is declining, according to the just-released World Drug Report 2014, put out by the U.N. Office on Drugs and Crime.

"However, in the United States, the lower perceived risk of cannabis use has led to an increase in its use," the report states. "At the same time, more people using cannabis are seeking treatment each year."

This included a 56 percent increase in US marijuana-related emergency room visits between 2006 and 2010, and a 14 percent increase in admission to drug-treatment programs over the same period, the report states.

According to the National Institutes of Health (NIH), about 9 percent of people who use marijuana become dependent on it – a number that increases to about one in six among those who start using it at a young age, and to 25 to 50 percent among daily users.

Meanwhile, the potency of pot has increased over the years due to genetic selection. "Daily use can have stronger effects on a developing teen brain than it did 10 or 20 years ago,” the NIH warns.

The UN cites the “risk of heavy dependence, lung problems, memory impairment, psychosocial development problems and mental health problems, and poorer cognitive performance associated with early initiation and persistent use between the early teenage years and adulthood.”

One of the arguments for marijuana legalization has been that, properly controlled by government agencies, it will provide a source of revenue for states. But the cost-benefit ratio may not be entirely clear, the UN reports.

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“Based on assumptions regarding the size of the consumer market, it is unclear how legalization will affect public budgets in the short or long term, but expected revenue will need to be cautiously balanced against the costs of prevention and health care,” the report states.

“In addition to the impact on health, criminal justice, and the economy, a series of other effects such as consequences related to security, health care, family problems, low performance, absenteeism, car and workplace accidents and insurance could create significant costs for the state,” the UN report cautions. “It is also important to note that legalization does not eliminate trafficking in that drug. Although decriminalized, its use and personal possession will be restricted by age. Therefore, the gaps that traffickers can exploit, although reduced, will remain.”