File photo: Elaine Thompson / AP

Los Angeles, California - Pot may be legal in eight US states and the capital, but tokers getting behind the wheel on a national weed appreciation day could be in for a downer as police forces crack down on intoxicated drivers.

In places such as California, where recreational marijuana has been legal since January 2018, law enforcement officials and doctors are worried about an increase in people arrested for erratic driving and hospitalised with overdoses.

The issue is particularly acute on 20 Apri, or 4/20, a national day of celebration in cannabis culture - especially after 4.20 pm when usage surges.

A recent study published in the Journal of the American Medical Association that looked at 25 years of data on fatal crashes in the United States found that the risk was 12 percent higher between 4.20pm and midnight on April 20, compared to the same time frame a week earlier and a week later.

The penalty for driving under the influence applies to both alcohol and other forms of drugs.

'Lack of knowledge'

John Hernandez, an eight-year veteran of the Los Angeles Police Department, said there was a serious "lack of knowledge on the users, so they still feel like 'I thought I can drive, it's all legal now'.

On a recent cool weeknight Hernandez pulled over a DUI suspect at a police checkpoint. The first thing he noticed was that the car reeked of pot.

The driver, a slim, well-dressed 20 year-old with bloodshot eyes, told Hernandez that he'd smoked some pot at noon, nearly nine hours earlier. His three friends in the vehicle, all about the same age, protested.

"He's OK, we're all OK," said the sole woman in the group. Hernandez was unconvinced, and put the suspect through a standard DUI roadside test: walk in a straight line, stand on one leg, touch your nose.

"I told him 'Put your hands on your side,' but he had his hands way out there, like an airplane," Hernandez said. "It looked like he's trying to fly away or trying to keep his balance."

Court date

The suspect said he was nervous, and that was causing his legs to be shaky - "but that's also a sign of someone who's under the influence of cannabis," Hernandez said.

The young man failed the test; Hernandez slapped on the handcuffs and sent him to a nearby bench to join other suspects waiting for a ride to the police station.

At the station the suspects are placed behind bars until the marijuana effects wear off. Next will come a court date, the likely loss of their driver's license, and the stain of a criminal conviction.

Aside from weed for smoking, marijuana dispensaries sell items like pot-based candy and chocolate. Experts warn that children could seriously overdose if they mistake the weed sweets for regular candy.

'Like a gun'

Mark Morocco, a professor and emergency care physician in Los Angeles, said doctors have seen a "gradual but progressive increase" in pediatric marijuana overdose cases since January.

Children in a pot haze usually just seem "altered," Morocco said. Doctors first have to rule out infection or trauma, the two leading causes of death or injury of pre-teen children. Then they have to consider "more rare and more insidious" possibilities, such as an overdose of prescription medicine.

A marijuana overdose is "the last thing we think about," he said.

Overdosing on pot is not lethal, and the treatment is rest until the effects wear off; in the case of a minor, a social worker must also be called in. Morocco tells parents to treat marijuana the same as prescription medicine, or "like a gun - you need to have positive control over it in your house".

Agence France-Presse