A blanket ban on so-called legal highs is now in force, amid questions over how far it will deter users and warnings it could drive dealers on to the "dark web".
Laws criminalising the production, distribution, sale and supply of the drugs took effect from midnight.
Offenders will face up to seven years in prison under the Psychoactive Substances Act. It had been widely expected that the measures would be rolled out in April but the start date was pushed back.
The legislation has come under intense scrutiny since it was first proposed by the Government last year.
Ahead of the ban, a survey by the YMCA suggested that while overall usage is likely to decrease, around two-thirds of young people who currently take the drugs are likely to continue using them in the future.
Meanwhile, there have been warnings that the ban could drive dealers on to the so-called "dark web" - unlisted websites that are difficult to trace.
New psychoactive substances - also referred to as designer drugs - saw an explosion in popularity on the drug scene in around 2008 and 2009. They contain substances which mimic the effects of "traditional" illegal drugs like cocaine, cannabis and ecstasy.
Official analysis published last month found that deaths linked to so-called "legal highs" more than tripled in two years, with a total of 76 recorded in England and Wales over a decade from 2004.
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."
The new Act states that a substance produces a psychoactive effect "if, by stimulating or depressing the person's central nervous system, it affects the person's mental functioning or emotional state".
A number of legitimate substances, such as food, alcohol, tobacco, nicotine, caffeine and medical products are excluded from the legislation.
Minister Karen Bradley said: "Too many lives have been lost or ruined by the dangerous drugs formerly referred to as 'legal highs'. That is why we have taken action to stamp out this brazen trade.
"The Psychoactive Substances Act sends a clear message -- these drugs are not legal, they are not safe and we will not allow them to be sold in this country."