Ozempic is a diabetes medicine that has been in high demand.
Falsified pre-filled Ozempic pens have been identified in the European Union and the United Kingdom, the European Medicines Agency (EMA) has warned.
The pens are labelled in German and originated from wholesalers in Austria and Germany, the regulator added.
"There is no evidence that any falsified pens have been dispensed to patients from legal pharmacies and there are no reports of harm to patients in relation to the falsified medicine," EMA said in a statement on Wednesday.
Authorities in impacted countries alerted wholesalers and pharmacies about the counterfeit pens.
Ozempic, sold by Danish pharmaceutical company Novo Nordisk, has become a blockbuster drug after being associated with weight loss.
It is currently authorised in the EU for the treatment of Type 2 diabetes, but there have been reports of it being prescribed "off label" for weight loss.
Its main ingredient is semiglutide, which is also used in the drug Wegovy, which can be used to treat obesity.
The high demand for the popular medicines has made Novo Nordisk Europe's largest company by market capitalisation. The company has also said its operating profit increased by 32 per cent at constant exchange rates in the first half of 2023.
The UK's medicines regulator warned that the falsified Ozempic pens had been identified at two UK wholesalers.
"All affected pens have been recalled and accounted for, and none of the pens have been supplied to UK patients," said the Medicines and Healthcare products Regulatory Agency (MHRA).
The pens were brought in from legitimate suppliers in Austria and Germany, the MHRA added.
The German drugs regulator (BfArM) said last week that while there was no evidence that the counterfeit pens had reached patients, it could not be ruled out that there are "in the distribution chain in Germany".
Ozempic pens have 2D barcodes and unique serial numbers on each package. While the counterfeit packs also had serial numbers, when they were scanned, those codes were inactive.
High demand for the Ozempic has led to a shortage, as demand outpaces the company's production.
A counterfeit Ozempic pen was found in the United States earlier this year, with Novo Nordisk warning that it appeared to contain a different diabetes medication.