"Three for a tenner," a man sings outside a south London nightclub. He's not selling anything but he's mimicking sellers of nitrous oxide balloons.
"Three for a tenner, we shot them all day long," he giggles as he walks into a small crowd at the bus stop.
It's not even 10pm on the last Saturday before the personal use of laughing gas is banned but these revellers are on their way home.
"I haven't done it for years," a woman says as she sits on a wall to rest her legs.
"Fully, fully welcome the ban," says another woman before a man chimes in: "It's not heroin, it's not crack, it's balloons - come on."
"Ban it," someone else shouts out. "Ban it, ban it. That's the one thing I agree that the government can do."
There are mixed reactions to the ban on recreational use of nitrous oxide which comes into place today.
Anyone using it to get high could face up to two years in prison, while sellers see potential sentences increased from seven to 14 years.
The new law follows two government-led reviews that declared the substance wasn't dangerous enough to merit a ban.
Laughing gas health risks
Nitrous oxide is used legitimately by the catering industry to whip up cream and as pain relief by doctors and dentists. Until this week, it was also widely touted as a party drug on the streets outside clubs and bars, attracting users of all ages.
Those who have experience of the party drug point to its numerous health risks.
It gives users a 90-second high but repeated misuse can lead to nerve damage, loss of eyesight and other health issues.
It is the second-most misused drug by 18 to 24 year olds across the country.
The government's ban, however, comes under plans to tackle anti-social behaviour and littering. Piles of discarded canisters litter the streets and have reportedly caused road accidents as drivers and cyclists swerve to avoid hitting them.
In announcing the ban in September, the home secretary had said "yobs" were abusing public spaces and "leaving a disgraceful mess for others to clean up".
Critics say the government has rushed through legislation to score political points ahead of an election year.
Leading neurologist in nitrous oxide abuse, Dr David Nicholl, said: "I am really worried that criminalisation of users will prevent people suffering from side effects from seeking health advice.
"This is already a very disenfranchised community, they are very hesitant to speak to doctors, so they may be even more reluctant knowing that use is illegal."
He said the feeling among medical professionals is that nitrous oxide should have been dealt with as a public health issue not an opportunity to criminalise youths.
Dal Babu, former assistant commissioner at the Metropolitan Police, says it will be a nightmare to police.
"It's going to be difficult for police to prove that a person is using it for recreational purposes," he told Sky News.
"It doesn't stay in your body for very long. It's not like cannabis or cocaine where you can do a test and find out if the drug is in a person's body," Mr Babu said.
Licences will not be required to carry nitrous oxide, but individual users will need to demonstrate they are lawfully in possession of it and not intending to wrongfully inhale it.
UK follows Netherlands lead
In banning the party drug, the UK follows the Netherlands which, in January, became the first country in the world to outlaw personal use of nitrous oxide after recording 1,800 car accidents over three years, resulting in 63 deaths.
The Dutch health minister who led the campaign to ban it told Sky News success comes from combining the ban with a preceding public education campaign.
"After 2018 we started with some preventative measures. These were targeted at schools, for example, and to youngsters using nitrous oxide," said Marten van Ooijen, Netherlands state secretary for health.
"We also advised that a ban on the recreational use of nitrous oxide was to follow.
"While we don't know exactly how much it's worked so far, what we've seen from 2018 until now is that the use of nitrous oxide has declined. For example, in 2018, 13% of youngsters were using nitrous oxide, but in 2022 that declined to 5%."
Sky News has been told that the Dutch ban has led to the creation of an underground laughing gas industry and an even bigger litter problem.
Many have stopped disposing spent canisters safely and instead throw them out with household waste, causing explosions and damage to waste management sites.
Last month the UK's policing minister Chris Philp appeared unprepared when Sky News asked him what plans were in place for police dealing with confiscated canisters.
Mr Philp pointed to the 13 tonnes of canisters collected by Kensington and Chelsea borough after the Notting Hill Carnival and admitted he had no idea how the council disposed of them.
The ban means street sellers and laughing gas hospitalisations could disappear, but the evidence of whether the government's ban is working will be in plain sight for everyone to see - discarded canisters on the streets could be a telling sign.