Amantadine and rimantadine are "adamantanes" or "M2" inhibitors .

Only effective against influenza A
Adamantanes are only effective against human influenza A, not influenza B. When used to treat seasonal influenza, they must be given to a patient within 48 hours of showing symptoms. They can shorten the duration of symptoms by about one day and reduce the amount of virus a patient sheds. They probably do not prevent complications or death from the flu.

Both M2 inhibitor drugs are about 70-90 percent effective as prophylaxis - preventing flu infection.

Influenza viruses can quickly develop resistance to these drugs when they're used to treat an infection. In January 2006, the Centers for Disease Control and Prevention (CDC) found that more than 90 percent of the circulating strain of seasonal influenza A/H3N2 tested had become resistant to M2 inhibitors. CDC issued an alert asking medical professionals not to use the M2 drugs amantadine and rimantadine to treat patients or prevent infection in the United States for the rest of the 2005-2006 flu season. This is an example of the limited use of these medications.

Resistance of A/H5N1
Some strains of avian influenza A/H5N1 ("bird flu"), including those which infected people in Thailand and Vietnam, are resistant to amantadine and rimantadine. Some strains in Russia, China, Indonesia and Turkey have shown susceptibility to amantadine.

Some countries have included adamantanes in their national medical stockpiles. However, they are not as widely stockpiled as other drugs because:

  • If the next pandemic is caused by an avian flu strain which is already resistant to adamantanes, the pandemic flu will probably also be resistant to these drugs.
  • When used for treatment, resistance rapidly develops.
  • Resistant strains can be transmitted to other people.
  • If a flu strain is resistant to one adamantane, it is resistant to the other.
  • These drugs commonly cause side effects.