An associate professor in paediatrics and adolescent medicine said on Tuesday that cannabis use by children can lead to damage of their central nervous system and early addiction.

Mike Kwan, from the University of Hong Kong, issued the warning following the admission of two very young girls to Tuen Mun Hospital on Monday, after they were allegedly fed candy containing cannabidiol. They were reported on Tuesday to be in a stable condition.

"It will cause essential nervous system problems. So the children, they may have loss of consciousness, a change in their behaviour and also ... agitation, and also some abnormal sensations and abnormal movements," Kwan said.

He added that cannabis users can also experience anxiety, and depression.

He said parents need to educate their children on the dangers of cannabis use, especially those who plan on studying in countries where the drug is legal.

"We need to be aware that in Hong Kong these products are, in fact, not licensed drugs; they are poison. Please keep Hong Kong, keep our home, free of these cannabis products ... although in overseas countries, these may be licensed products," Kwan said.

"Do not import these products into Hong Kong and most importantly do not keep these products at home. This is the most important way to avoid our children [coming] into contact with these products."