Paediatricians, poison control centres and road safety groups warned American parents on Saturday to check Halloween sweets for cannabis edibles and keep children visible near traffic as millions went trick-or-treating across the United States. The warnings came on one of the deadliest nights of the year for child pedestrians.

America's Poison Centers said it had recorded a steep rise in calls involving young children who swallowed THC-infused gummies and chocolates. Many are packaged to resemble popular branded sweets.

The US Food and Drug Administration has repeatedly cautioned that copycat edibles can be easily mistaken for ordinary candy, and the American Academy of Pediatrics advised parents to sort through children's haul before anything is eaten.

Road safety was the second major concern. A study published in JAMA Pediatrics found that pedestrian deaths rose by roughly 43 per cent on Halloween evenings compared with other nights.

Safe Kids Worldwide said children were more than twice as likely to be struck and killed by a vehicle on Halloween as on any other day. The National Highway Traffic Safety Administration urged drivers to slow down in residential areas and avoid driving after drinking.

Emergency physicians said the risks were compounded this year by the timing of the clock change. Daylight saving time ends in most of the United States at 2am on Sunday, meaning sunset will arrive an hour earlier from next week. The American College of Emergency Physicians urged families to use reflective tape, torches and glow sticks, and to choose flame-resistant costumes to reduce the risk of burns from candles in pumpkins.

Local health departments also flagged the spread of synthetic drugs. Erie County officials in Pennsylvania said this week they were monitoring cychlorphine, an emerging synthetic opioid documented by federal investigators in several states. Poison control specialists advised parents to call the national helpline at 1-800-222-1222 immediately if a child showed drowsiness, vomiting or confusion after eating Halloween treats.