The World Health Organization says relatively few cases of Tamiflu resistance have been found among pandemic H1N1 viruses, but there are some concerns that bear watching.
The agency says there have been 39 cases of Tamiflu resistance reported so far, seven of which are still being investigated.
Of the 32 for which details are known, 13 occurred in people taking the drug to prevent infection, a procedure called prophylaxis.
The WHO says while the numbers are low, resistance hasn't previously been seen in people taking the drug prophylatically and the fact it is now being seen is a concern.
And there have been three cases of drug resistance in people who hadn't taken Tamiflu, which is the brand name for oseltamivir.
The WHO says there's no evidence of ongoing spread of resistant viruses, but there is a strong suspicion that limited spread has occurred among close contacts in some cases.
Most of the viruses have been found in North America and in the Western Pacific region, though a few have been spotted in Europe.