GPs have been put on alert over fears that Tamiflu can put some people at greater risk of suffering a stroke.
A Government watchdog is concerned that the anti-swine flu drug can interact with the blood-thinning medication warfarin, which is taken by more than 600,000 people in the UK.
The combination can dangerously thin the blood, putting patients at risk of uncontrolled bleeding which can lead to a stroke.
The Medicines and Healthcare products Regulatory Agency has already received reports of such cases and has asked health professionals to watch out for more.
Last night an expert warned that the dangers have been increased because people given Tamiflu over the national flu hotline are not being warned properly about the possible warfarin risk.
The MHRA has now received 418 reports of suspected adverse reactions to Tamiflu, including two deaths.
