The EU's medicines regulator said it is investigating reports of possible "acute" kidney damage in patients taking the coronavirus drug remdesivir, made by US pharma giant Gilead.
The European Medicines Agency recommended the use of the anti-viral drug, sold under the brand name Veklury, in June after studies showed it could reduce the length of hospital stays for Covid-19 sufferers.
"EMA's safety committee has started a review... to assess reports of acute kidney injury in some patients with Covid-19 taking Veklury (remdesivir)," it said in a statement.
The regulator said it had "not been determined whether there is a causal relationship" between remdesivir and kidney problems, but that the issue "warrants further investigation."
Covid-19 itself could also cause kidney injuries, the regulator noted.
Recommendations for using the drug had not changed for now, the EMA said, adding that approval had been fast-tracked for use with coronavirus patients on the basis that further tests would be carried out.
If a link to kidney problems was proved, any change would probably be limited to making leaflets with the drug alerting users to possible side-effects, it said.
The World Health Organization said remdesivir was "still being tested in trials".
"We are in very close contact with the European Medicines Agency and we follow any investigations that they are doing," said Emer Cook, director of pre-qualification and regulation at the WHO.
"As far as I understand, this investigation is at an early stage of development, but we will be in touch and make sure that we follow up and liaise in terms of the trials that are ongoing."