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Marijuana laced with fentanyl is rising across the US, a Washington doctor has warned, after cases of the drug cocktail were found in New York, Alabama, Illinois and Louisiana.

Fentanyl, a highly potent synthetic opioid that's 100 times stronger than morphine and 50 times stronger than heroin, is increasingly being mixed with other substances to make them more potent at a cheap price.

Just two milligrams of fentanyl – imagine 10 to 15 grains of table salt – can prove fatal, and staggering figures showed it is killing the equivalent of an entire classroom of children every week.