Possessing canisters of nitrous oxide, known as laughing gas, to get a psychoactive high is now illegal in Britain.
The sale of small canisters has escalated in recent decades, especially among youngsters, who are looking for a quick high.
The ban was first proposed by Prime Minister Rishi Sunak's Conservative government in March and is now imposed under the 1971 Misuse of Drugs Act.
Serial offenders can now face up to two years in prison and dealers up to 14 years.
British law already bans the "knowing or reckless" supply of nitrous oxide for inhalation outside of medical uses.
But the canisters have become available online and on the streets.
Policing minister Chris Philp said for far too long, the use of this drug in public spaces has contributed to anti-social behaviour.
Prolonged use can cause anaemia, nerve damage and spinal injuries, doctors warn, while the canisters have come to litter parts of urban and suburban Britain.
