Auckland City Missioner Diane Robertson has said her staff have reported a rise in clients showing symptoms of solvent abuse.
However, it was too early to indicate it is linked to withdrawal from synthetic highs, as many of her clients have stockpiled the drugs, she said.
Mrs Robertson said she is monitoring a group of clients who were users of the psychoactive substances that were taken off shelves in an urgent legislation change that came in to effect last Thursday.
All psychoactive substances have been banned until a government-approved testing regime is in place to ensure their safety.
"We are monitoring to see what the effects of that are on them and their behaviour, and if there are any issues around withdrawal, and whether there is an increase in alternative substances," Ms Robertson said.
She said people had stockpiled the substances before they were banned, so it could be weeks before they start to come off the drugs.
Alcohol Drug Association New Zealand chief executive Paul Rout said people looking for an alternative to synthetic highs could very well turn to solvents.
"When a drug that was legal and relatively cheap goes off the market then people will turn to other alternatives in many cases," he said.
"[Solvent] is probably not too expensive and you can pick it up because there's always a few retailers who may not be as scrupulous around who they sell it to as they should be."
He had not heard of solvent abuse as a specific alternative to legal highs.
Christchurch City Missioner Michael Gorman said he was expecting an increase in solvent abuse once stockpiles run low.
"It's a bit too soon, but we are preparing for it," he said.
Wellington City Mission chief executive Michelle Branney said similarly, users of synthetic highs had stockpiled and it was only a matter of time before people were withdrawing.
"We haven't seen that, not yet. We are on stand-by," she said.
Some of the country's busiest emergency departments had not seen any increase in the number of patients with solvent abuse problems.