BZP is one of the drugs sold as an alternative to ecstasy
Users of legal recreational drugs will be banned from importing their own supplies into Guernsey, the Health and Social Service minister has said.
The States banned the commercial importation of the herbal drugs in April following health concerns.
More than £400,000 has been spent so far this year on sending about 30 users to the UK for treatment.
Deputy Hunter Adam said a problem was a perception that the products were safe, but that has not been proven.
He said: "There is unknown content in these 'herbal' drugs - you've got unknown effect and you've got unknown harm.
"Now we're not calling them legal highs, we're calling them 'emerging drugs of concern' because that is what they are."
He said the ban on personal importation would be in place shortly as legislation is going through to stop the "loophole that just makes it commercial".
Deputy Adam said the worse effect was psychosis: "Mental problems in their behaviour and their perception of their behaviour and how they are coping with life in general.
"In fact they are not coping, they are either going very manic or depressive and becoming psychotic in manner and behaviour towards other people.
"It's a wide spectrum of mental problems they're experiencing due to the effect of drugs."
He said it was addition strain when budgets are tight: "We have a budget for off-island treatment, if it suddenly increases by £500,000 (his estimate of the spend across the whole of the year) on one group you can understand how it will go over budget."