Doctors have been alerted over fears that Tamiflu could put some patients at increased risk of suffering a stroke, it was reported today.
The British Medicines and Healthcare products Regulatory Agency (MHRA) is concerned that the drug can interact with warfarin, which is taken to thin the blood.
The combination may affect the rate at which the blood clots, putting some patients at risk of uncontrolled bleeding which can lead to a stroke.
The MHRA said they were monitoring the situation closely over the concerns.
Doctors measure how easily the blood clots using the INR rate, and the higher the INR rate, the thinner the blood is.
An MHRA spokesman said: “We have seen indications that INR rates could possibly rise due to interactions between warfarin and Tamiflu.
“However, flu-like illnesses have also been known to cause this, so at this stage it is difficult to know whether it is the interaction with Tamiflu or the underlying flu.”
The MHRA added: “We‘ve had suspected reports sent to us before the pandemic and we’ve had reports sent to us during the pandemic and it is one of the issues that we are monitoring very closely.”
A British government spokeswoman said: "Anyone who is already taking medication for an underlying health condition is advised to contact their GP practice by phone before starting a course of Tamiflu.
"The department is working closely with the MHRA to ensure that all possible interactions between Tamiflu and other prescription drugs are monitored."
It is the latest concern to emerge about Tamiflu, following warnings last week that children should not be given the drug.
Oxford University researchers said that children should not be given the anti-viral drug as youngsters who were otherwise healthy could suffer more harm than benefit from taking Tamiflu or another anti-viral, Relenza.