So are 'legal highs' now illegal?
The government say it is a "landmark law" that will "fundamentally change the way we tackle these drugs" but critics say it will have little effect.
Anyone caught dealing or making "psychoactive substances" now faces up to seven years in prison.
That is defined as anything you can take which has "a direct effect on mental processes" but alcohol, nicotine and caffeine are all exempt.
Possession of small amounts of the substances is still not a crime.
So that has left some people wondering - are legal highs still legal?
What is banned?
The government has always insisted that this new law will "put an end to the game of cat and mouse in which new drugs appear on the market more quickly than government can identify and ban them."
It says "in 2014 alone, psychoactive substances were involved in the deaths of 144 people in the UK up from 31 deaths in 2010" and that's why it's bringing in the law.
Now, you will be breaking the law if you make or sell the substances.
If you are caught by the police with a small amount of new psychoactive substances (NPS) you're not going to get arrested.
That will only happen if you have enough on you for the police to believe you plan to supply it.
Edward, 19, from Leeds regularly uses a legal high called Spice.
We met him as he was stocking up on legal highs ahead of the ban. He told us:
"The shop shut today. Now I've got four packs in my pocket.
"I'm addicted to this stuff. I've been smoking it for five years straight."
Edward says people in the area have been buying in bulk ahead of the selling ban, which applies across the UK.
Dealers he knows are trying to make similar substances at home themselves.
"You can be like a smack head on it, if you haven't got the right mind set."
Will it work?
Some legal experts argue that it will be hard to implement because it's difficult to prove when a substance is psychoactive.
Stephen is 22 and was addicted to synthetic cannabis for two years, he says the users he knows will carry on taking it.
"I don't believe the law will work, I think you need to ban it for everyone.
"They should arrest people for possession and not just give them a slap on the wrist."
Youth charity the YMCA has been doing some research and it reckons overall usage is likely to decrease.
However it says there will still be two-thirds of young people currently taking legal highs that are likely to continue to use them in the future.
It's done a survey of more than a thousand 16 to 24-year-olds across England and Wales.
Denise Hatton is the chief executive of the charity and she says many long term users will find a way around the ban.
"Unless the act is backed up with changes to the education and support currently available, our evidence says that these young people and more will continue to experience harm at the hands of legal highs for years to come."
Has it worked elsewhere?
This law is very similar to the Irish system and when Newsbeat visited Ireland we were told that it had seen limited results.
Drug workers and users told us that it is easier to get the drugs since the ban was brought in in 2010.
Looking at the figures on this would also suggest that the Irish system has struggled.
To the end of 2015 there have only been four successful prosecutions and experts say recently they have seen a worrying rise in use.
Anna, 21, is a student in Leeds. She's tried laughing gas before and told us the new law would put her off:
"It could be good to stop dangerous legal highs but bad for people who use them safely.
"It would probably stop me as I wouldn't want to do anything illegal."
Will it send the sale of new psychoactive substances underground?
That's certainly the fear of the charity Addaction.
"There are concerns that the act could potentially push these so called 'legal highs' underground and we could see them emerging in more street deals and potentially being used as cutting agents for illegal drugs," it told Newsbeat.
Jodie Robertson is from the YMCA - she works with teenage drug users in Sunderland, she says the new law won't stop them getting hold of the substances because they will go online internationally.
"How can you expect to police the internet?
"The drug dealers are going to be able now to get their hands on this stuff, cut it with whatever other drug they want, repackage it and sell it on."
The government minister in charge of this law is Karen Bradley.
"Psychoactive substances shatter lives and we owe it to all those who have lost loved ones to do everything we can to eradicate this abhorrent trade, she said.
"The message is clear so-called 'legal highs' are not safe. This act will ban their sale and ensure unscrupulous traders who profit from them face up to seven years in prison."
There's more information here if you want to get help or advice about legal highs.
And you can look at these BBC Advice pages on drug use here.
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