Over the festive period two deaths were reported as a result of the drug use, and on Friday another two cases were confirmed as being linked to the drug.
“Four people are dead from the drug,” Towe Hägg a press officer for Stockholm police told The Local.
“The common denominator for all cases has been the Superman pill,” Annika Ljungberg, an inspector at Stockholm police, said to the TT news agency.
"This indicates that all the victims used and died from using these tablets."
Police and Sweden’s Poison Information Centre (Giftinformationscentralen) are warning Swedes not to take the pills, which are illegal.
The tablets – which are usually pink or green with a 'Superman' logo on them – contain a chemical known as PMMA, which works in a similar way to MDMA, the main ingredient in ecstasy. But it can take longer for users to get high.
The drug – which has also gained the nickname ‘ugly ecstasy’ in Sweden – is cheaper to produce than other ecstasy tablets and its usage also appears to be spreading quickly in several other European countries including the UK and the Netherlands.
“If you have a pink tablet with a Superman logo, you should definitely get rid of it,” Erik Lindeman from the Poison Information Centre in Sweden told TT.
“It's a scam,” he added.
The Poison Information Centre is urging anyone in possession of the pills to hand them in to police.
Four deaths in Britain were linked to the drug between Christmas and New Year and people in Canada and Israel are also understood to have died after taking the pills.