By Phil Mercer, BBC News, 5 August 2011

A 12-month ban of synthetic cannabis products has come into effect in New Zealand.

The government says the 43 artificial cannabis products can harm health, including causing heart problems

But consumers of the products say the ban will force them to buy recreational drugs on the black market.

Among the prohibited substances is a popular brand known as Kronic, which has been a legal alternative to marijuana for several years.

It is smoked like marijuana and has been available in shops and tobacconists.

Some retailers believe that other legal highs will continue to flood a very popular, and lucrative, market.

Chris Fowlie, manager of the Hemp store in New Zealand's largest city Auckland, says there will be ways for suppliers to circumvent the new regulations.

"I think this will encourage companies to just put out one product at a time, you know, maximise their sales for it and then when that gets banned simply replace it with the next one," he said.

"We already know that there are hundreds if not thousands of synthetic cannabinoids out there."

But Peter Dunne, New Zealand's associate minister for health, says that if new products become available they too could be banned.

"The temporary class drug notice will be able to be made available to any psycho-active substance where there is a risk, or a perceived risk or an apparent risk to, if you like, human safety," he said.

The new law comes into place on Friday and stockists have until the middle of the month to remove the products from their shelves.

The ban will initially run for 12 months.

Next year New Zealand's conservative government plans to bring in comprehensive changes based on a review of the country's drug legislation by the Law Commission.