Lambeth Council in London has become the first in the UK to ban the use of nitrous oxide, or laughing gas, as a recreational drug.
Although medically safe and not illegal, the chemical can cause a drop in blood pressure, fainting and heart attacks when used on a regular basis.
But those using it publicly in Lambeth can now be issued with an on-the-spot fine of up to £1,000.
Councillor Jane Edbrooke said using the gas was "not harmless fun".
The Global Drug Survey says it is the UK's fourth popular recreational drug.
In 2013-14, some 470,000 people took nitrous oxide, according to the Home Office.
It is especially popular with young people, with 7.6% of 16 to 24-year-olds taking it that same year - a greater proportion than took cocaine (4.2%) and ecstasy (3.9%).
There have, however, been 17 fatalities related to the use of laughing gas in the UK between 2006 and 2012, according to research.
The Advisory Council on the Misuse of Drugs says there was one death in 2011 and five in 2010. The US records about 15 deaths a year.
It is illegal to sell to under-18s if there is a risk they will inhale it, but it remains available for adults to buy.
Lambeth Council has now used new powers allowing local authorities to make their own laws to tackle local problems.
Ms Edbrooke said the council hoped the ban would get a message across to the public.
"It's not healthy, just because they're caused legal highs does not mean they're good for your health," she said.
Local police and council officers will both enforce the ban.