Chicago ? As flu season hits high gear, doctors and health professionals say treating cases is more complicated than ever because this year’s most common strain has developed resistance to the drug known as Tamiflu.

The drug is one of only four antiviral medicines available to battle influenza. Scientists from the agency co-authored a paper on the resistance, which was published Monday in the Journal of the American Medical Association.

Because Tamiflu is no longer effective against the season’s main flu strain, known as H1N1, physicians are turning to the other three antivirals, all of which have downsides.

Zanamivir, an inhaled drug known as Relenza, is not recommended for some of the very populations that would need it most: very young children and people with respiratory problems. Rimantadine, sold as Flumadine, and amantadine, sold as Symmetrel, have resistance troubles of their own.

Health officials are urging people to get a flu shot, which they say offers good protection this year against the H1N1 strain.