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Community Alcohol and Drug Service medical director Dr Gavin Cape said regular users of legal highs might only feel ''normal'' on the drug and, when they stopped using, their body and brain had to adjust.
The withdrawal was the brain getting used to working ''normally'' again without the drug, Dr Cape said.
Most users would have mild, or little withdrawal symptoms for a few days, such as anxiety, disturbed sleep, irritability, poor concentration, cravings, nausea, aches and little appetite.
The advice to deal with withdrawals included gentle exercise, drinking plenty of water and to take it easy, knowing it was a natural response and would end soon.
People with more severe withdrawal symptoms, such as severe vomiting, diarrhoea, shakes and tremor, racing or irregular heartbeat or thoughts of self-harm should call their doctor.
If users hallucinated, had delusions and psychosis, seizures or loss of consciousness they should seek emergency medical attention.
A 30-year-old synthetic cannabis smoker, of South Dunedin, who did not want to be named for fear he would lose visitation rights to his daughter, said he smoked legal highs to replace methamphetamine.
''It's the same buzz ... I used to be a heavy meth smoker until this [legal highs] came along; it got me off a lot of other drugs.''
When legal highs were banned, he would return to those drugs, including methamphetamine, which were readily available in Dunedin, he said.
He had briefly given up smoking synthetic legal highs before and there was no easy way to detox.
''You've just got to tough it out.''
Cupid Shop owner Carl Lapham said about 95% of his customers yesterday were buying legal highs and many were stockpiling as much as they could afford.
The sale of legal highs had already gone ''underground'' in Dunedin and customers had reported people selling legal highs from a shoebox in the Octagon at the weekend, he said.
A Dunedin police spokeswoman said police were unaware of those claims, but if police received any information about such happenings it would be followed up.
Health Ministry spokesman Kevin McCarthy said although the amended Bill had not yet been passed, it was expected to be operative from 12.01am tomorrow.
Where to get help
• The Alcohol Drug Helpline: 0800 787-797.
• The Addictions Treatment Directory: www.adanz.org.nz
• Home-treatment-based Detox Service: 476-6063.