A cult Trader Joe’s seasoning that has become a favourite souvenir for South Koreans returning from the US has come under police investigation after authorities blocked its resale on a popular online marketplace.

The Mapo police station in Seoul said officers discovered people selling Trader Joe’s Everything But the Bagel Sesame Seasoning Blend on Karrot, where it had become a popular resale item after travellers brought it back from the US, according to Yonhap.

After forensic tests detected poppy seeds along with “trace amounts of morphine and codeine”, police warned that anyone buying or selling the product could face criminal punishment.

Karrot, one of South Korea's largest secondhand marketplace platforms, has since removed listings for the seasoning from public view, stepped up monitoring, and introduced a “Transaction Restricted” warning that appears whenever users attempt to list the product for sale. The measures were introduced after police requested the company block further transactions.

According to the police, some people had listed the seasoning without realising it contained ingredients classified as narcotic substances under South Korean law, while others attempted to sell it despite knowing the restrictions.

Authorities are investigating those involved on suspicion of violating the Narcotics Control Act.

“Even if a product is legally sold overseas, it may be classified as a narcotic substance or a prohibited import in Korea, so particular caution is required,” a Mapo police station official told The Korea Herald.

At the heart of the dispute are the poppy seeds used in the seasoning.

Poppy seeds are harvested from the opium poppy, whose latex is used to produce medicines including morphine and codeine. Although the seeds themselves do not naturally contain those narcotic compounds, they can pick up trace residues during harvesting.

Poppy seeds are a staple of breads and pastries across Europe and are also used in Indian cooking to thicken curries and spice pastes, making South Korea's strict restrictions an unexpected trap for many overseas travellers.

The police urged consumers to check the relevant regulations before selling or purchasing food products brought into the country from abroad, adding that authorities would “respond strictly” if products containing narcotic substances continued to circulate.

The viral seasoning, made of sesame seeds, dried garlic, dried onion, sea salt, black sesame seeds, and poppy seeds, has been banned from entering the country since 2022 but only attracted widespread international attention two years later.

In July 2024, travellers arriving at Seoul’s Incheon airport began reporting that customs officers were confiscating jars of Trader Joe’s Everything But the Bagel seasoning from their luggage, with photos of airport warning signs quickly spreading across social media, according to NBC News.

The US embassy in Seoul also warned travellers at the time that “food products containing poppy seeds, including popular bagel sesame seasoning blends, are considered illegal in Korea”, while customs officials said they had stepped up inspections of products linked to narcotics.

For many South Koreans travelling to the US, the seasoning had become an easy purchase alongside Trader Joe’s increasingly coveted tote bags. The blend earned a devoted following thanks to social media and years of strong customer demand, regularly ranking among the supermarket chain’s favourite products.

Before police stepped in, bottles were being resold on Karrot for between 4,000 (£2) and 8,000 won (£4).