A blanket ban on so-called legal highs has come into force in the UK.
Laws criminalising the production, distribution, sale and supply of what are otherwise known as new psychoactive substances began at midnight.
The chemicals, sold under names such as spice and black mamba, are designed to give users the same effect as drugs like cannabis and cocaine.
Last year legal highs were linked to more than 100 deaths in the UK and a rise in violent assaults in prison.
Offenders who break the new laws will face up to seven years in prison under the Psychoactive Substances Act.
Police will also be able to shut down "headshops" - stores which sells drug paraphernalia - and online dealers in the UK.
However, there have been warnings the ban could drive the sale of the drug to the so-called "dark web" - a largely untraceable area of the internet that does not show up on traditional search engines.
A survey by the YMCA charity - conducted ahead of the ban coming into force - also suggested two-thirds of young people who currently take the drugs are likely to continue using them in the future.
Under the new legislation, authorities will have powers to seize and destroy psychoactive substances, as well as carry out searches of people, premises and vehicles.
If a person is found to be in possession of a psychoactive substance in prison, they could face having up to two years added to their sentence.
The legislation has come under intense scrutiny since it was first proposed by the government last year.
It had been widely expected that the measures would be rolled out in April but the start date was pushed back.
'A ban on getting high'
Analysis: BBC home affairs editor Mark Easton
The story of prohibition over the last 100 years is not a particularly happy one.
Banning alcohol in the US in 1920 pushed the trade underground, with criminals selling bootleg booze of dubious or dangerous quality.
Some 10,000 people died from drinking poisonous liquor before prohibition was lifted.
Many argue the so-called "War on Drugs" has done the same: Simply handing the trade to unscrupulous international criminal gangs.
Drug deaths in Britain are currently at record numbers. But with no governmental appetite in Britain for decriminalisation or legalisation of drugs, prohibition remains the only response the Home Office is prepared to consider.
Simon Blackburn, of the Local Government Association, said legal highs were a "scourge on society and shatter lives".
He added the new blanket ban "should help to reduce anti-social behaviour" linked to their use.
"Councils have made every effort to crack down on these substances and the unscrupulous traders selling them, which has seen so-called 'head shops' closed down, intoxicating substances seized, on-the-spot fines issued and successful prosecutions.
"However, this work relied on laws designed for very different purposes, making it much harder for councils and the police to tackle the problem."
Edmund Smyth, criminal lawyer at Kingsley Napley, said police "have ever-more stretched resources so questions remain about their ability to enforce the new regime effectively".
He added: "Many have criticised this act in draft stages - it may prove to be a sledgehammer to crack a nut and have unintended consequences. But it is here and carries serious consequences for those who fall foul of the new law."
Campaigner Karen Vandersypen - whose son Jimmy died in 2014 after taking a legal high - said she was "delighted" the ban had become law.
Jimmy, 20, had a heart attack and later died after taking synthetic cannabis from a shop in Kent.
"We are absolutely delighted this has come into effect," she told BBC Radio 4's Today programme.
"It is just such a shame we have paid such a high price for having it there."
The ban comes into force a day after the owner of a shop selling legal highs and a shop worker were arrested in Greater Manchester after nine people fell ill after taking substances.
The legal highs now illegal
Some of the legal highs which have hit the headlines in recent years include:
- Spice - Replicating the doping effect of cannabis, spice is one of the brands which has gained notoriety following reports of its widespread use in prisons. It comes as a smoking mix and has been known to cause paranoia, delirious ranting and hallucinations. Similar drugs go by the names black mamba and annihilation
- Laughing gas - Otherwise known as nitrous oxide, laughing gas comes in canisters and is used recreationally after being inhaled, often out of balloons. It gives users a light-headed, euphoric feeling that lasts for several seconds, but, due to it depriving the body of oxygen, can be fatal when taken in excess
- Salvia - Unlike other synthetic legal highs, salvia comes from a plant. It is still sold in many so-called headshops, but only on the proviso that it is not marketed for human consumption. When smoked or chewed, it can create a hallucinogenic experience
- Mephedrone - The drug which also goes by the name "mcat" and "meow meow" shot into the public spotlight in 2010 following a string of deaths. It mimics the effects of many amphetamines such as speed and MDMA, providing similar feelings of elation, but with a potentially deadly impact on the heart and central nervous system. It was outlawed within months and is currently a class B drug