A former legal-highs user is predicting a "massive black market" now they are illegal.

At 12.01am today an amendment to the Psychoactive Substances Act, which Parliament passed on Tuesday night, came into effect, making it illegal to manufacture, sell and possess all synthetic cannabis products.

The products have to be pulled from shelves until they can be proven safe. The amendment also bans the use of animal testing to get products approved.

A former legal-highs user, who did not want to be named, told the Chronicle he agreed with the law change but warned there would be consequences.

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"There's going to be a massive black market for it straight away," he said.

He was now clean but said that because the drugs were no longer available, users would experience headaches, sweats and paranoia.

However, Whanganui DHB Community Mental Health and Addiction Services clinical nurse manager Sharon McFarland did not expect it to be any more of an issue.

Help was on hand, she said.

"We are not expecting an increase in people presenting to our Community Mental Health and Addiction service, we see it as business as usual. [But] there may be an increase in people enquiring about what our service can provide."

One of those licensed to sell the legal drugs, Avenue Choices owner Alois Muller, yesterday expected to be almost out of the product by the end of the day. He had stopped ordering when news of the Government's intention to change the law broke.

Mr Muller said it had not been particularly busy despite it being the last chance to buy the drug legally and predictions of stockpiling.

He was "not 100 per cent" happy with the law change and said it would drive the product on to the black market.

"I believe they should have thought about it a little bit further because I feel like [there will be] more problems coming up, this is just my opinion."

He believed the issue was a "beat-up". Many people could handle the product, he said.

The ones who could not were the exception: "It's the same with alcohol. A lot of the customers we have here, they can cope with it."

Possessing a small amount of a psychoactive substance will earn a $500 fine.