The antiviral drugs amantadine and rimantadine inhibit a viral ion channel (M2 protein), thus inhibiting replication of the influenza A virus.[86] These drugs are sometimes effective against influenza A if given early in the infection but are ineffective against influenza B viruses, which lack the M2 drug target.[160] Measured resistance to amantadine and rimantadine in American isolates of H3N2 has increased to 91% in 2005.[161] This high level of resistance may be due to the easy availability of amantadines as part of over-the-counter cold remedies in countries such as China and Russia,[162] and their use to prevent outbreaks of influenza in farmed poultry.[163][164] The CDC recommended against using M2 inhibitors during the 2005–06 influenza season due to high levels of drug resistance.[165]