A trading standards expert has warned online stores need to "take responsibility" to prevent the illicit sale of nitrous oxide.
The gas - dubbed "laughing gas" or "nos" - is being sold with equipment needed to take it as a high on sites like Amazon and eBay, BBC Wales found.
It is the second most commonly used recreational drug in England and Wales after cannabis.
Sales are tricky to police as it has legal uses in catering and pain relief.
Nitrous oxide is sold on Amazon and eBay alongside the "crackers" and balloons used to take it.
On social media sellers were happy to deliver around the clock.
Samantha - not her real name - used nos when she was younger after a housemate bought it on Amazon.
The 22-year-old from Cardiff said: "When you're that age and everyone around you is doing it, and you're not really seeing any bad, negative impacts from it, you think, 'Oh it's fine, it's something that young people do'."
But she experienced fizzing in her nose, nausea and a tight chest after taking a substance friends bought online.
They thought it was nos. It was CO2.
Carbon dioxide is not used recreationally but inhaling it carries similar risks.
"The next day I felt really, really terrible, and I think it was a lot of anxiety about what I'd done the night before," Samantha said.
"It was something that turned me off doing anything like that because it was so scary."
Nitrous oxide - the highs and lows
- Effects include euphoria, calmness, dizziness, difficulty thinking straight, giggling and hallucinations
- Inhaling nitrous oxide from the canister or in an enclosed space - like with a bag over your head - is very dangerous
- By inhaling nitrous oxide the user risks falling unconscious or suffocating from lack of oxygen. People have died this way
- If someone collapses after using nos, turn them on to their side, call 999 and stay with them until an ambulance arrives
Nitrous oxide has been linked to 17 deaths in the last three years, according to official statistics. Among 16 to 24-year-olds about one in 11 used it last year.
Legislation introduced in 2016 made it illegal to sell as a high.
Prosecutors say the law is not working because its legal uses make enforcement tricky.
BBC Wales found boxes of nos canisters being sold on Amazon in a special deal including the balloons used to take it.
On eBay, some "crackers" were sold alongside balloons. There were money-saving deals on bulk purchases and nos canisters advertised in the "similar sponsored items" section.
When BBC Wales searched for nitrous oxide canisters on both sites, crackers and balloons also came up in searches and were suggested by the sites' algorithms as products that could be bought with nos.
Amazon has since removed the product being sold as a package of nos canisters and balloons.
The company said sellers must follow their guidelines.
A spokesman said: "Those who do not will be subject to action including potential removal of their account."
An eBay spokesman said: "Listings encouraging illegal activity are banned from eBay's platform.
"We have removed the items and are taking enforcement action against the sellers."
Nitrous oxide is also sold through social media accounts.
Many carry warnings against recreational use, but when a BBC Wales investigator called five sellers in Wales and south-west England, all were happy to deliver nos that night - despite the reporter saying it was for recreational use.
Some offered discount deals for multiple boxes.
Caerphilly council's Tim Keohane secured one of Wales' first prosecutions of a shop for illegally selling it in August.
Caerphilly and Gwent Police prosecuted Khehra Store Ltd after it was found to have sold nos at the 7-11 shop in Bedwas Road, Caerphilly, in 2018.
The firm and its boss were hit with fines and charges of about £2,000.
Anyone found guilty of selling or giving away nitrous oxide for illegal purposes can face up to seven years in prison, an unlimited fine, or both.
Mr Keohane said the offence was harder to prove with online vendors. They can flout the law by selling items separately or posting disclaimers against misuse.
He said the drug's widespread use among the young and online sales were a "huge concern".
But its legitimate uses - such as for producing whipped cream - made legislating against web distribution difficult.
Mr Keohane said: "Companies like Amazon and eBay need to take responsibility because it is so difficult to police the internet and sellers."
The Royal College of Nursing (RCN) has said many are unaware of the risks.
They include breathing difficulties, increased heart rate, burns and death.
Mental health nurse Jeremy Davis, of RCN Wales, said: "For every young person who has a balloon at a party and has five minutes that are the best of their evening, there is another one who wakes up in A&E.
"There are four or five more [each year] who don't wake up."
In May, several 4ft cylinders were stolen from Prince Charles Hospital, Merthyr Tydfil.