Influenza viruses can show resistance to anti-viral drugs. Like the development of bacterial antibiotic resistance, this can result from over-use of these drugs. For example, a recent study published in the June 2009 Issue of Nature Biotechnology emphasized the urgent need for augmentation of oseltamivir (Tamiflu) stockpiles with additional antiviral drugs including zanamivir (Relenza) based on an evaluation of the performance of these drugs in the scenario that the 2009 H1N1 'Swine Flu' neuraminidase (NA) were to acquire the tamiflu-resistance (His274Tyr) mutation which is currently widespread in seasonal H1N1 strains. Yet another example is in the case of the amantadines treatment may lead to the rapid production of resistant viruses, and over-use of these drugs has probably contributed to the spread of resistance. In particular, 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, and their use to prevent outbreaks of influenza in farmed poultry.
On the other hand, a few strains resistant to neuraminidase inhibitors have emerged and circulated in the absence of much use of the drugs involved, and the frequency with which drug resistant strains appears shows little correlation with the level of use of these drugs. However, laboratory studies have shown that it is possible for the use of sub-optimal doses of these drugs as a prophylactic measure might contribute to the development of drug resistance.
During the United States 2005–2006 influenza season, increasing incidence of drug resistance by strain H3N2 to amantadine and rimantadine led the CDC to recommend oseltamivir as a prophylactic drug, and the use of either oseltamivir or zanamivir as treatment.
Antiviral drugs are prescription-only medication in the United States. Readily available over-the-counter medications do not directly affect the disease, but they do provide relief from influenza symptoms, as illustrated in the table below.
Children and teenagers with flu symptoms (particularly fever) should avoid taking aspirin as taking aspirin in the presence of influenza infection (especially Influenzavirus B) can lead to Reye's syndrome, a rare but potentially fatal disease of the brain.
Malnutrition can reduce the ability of the body to resist infections and is a common cause of immunodeficiency in the developing world. For instance, in a study in Ecuador, micronutrient deficiencies were found to be common in the elderly, especially for vitamin C, vitamin D, vitamin B-6, vitamin B-12, folic acid, and zinc, and these are thought to weaken the immune system or cause anemia and thus place people at greater risk of respiratory infections such as influenza. Seasonal variation in sunlight exposure, which is required for vitamin D synthesis within the body, has been proposed as one of the factors accounting for the seasonality of influenza. A meta-analysis of 13 studies indicated some support for adjunctive vitamin D therapy for influenza, but called for more rigorous clinical trials to settle the issue conclusively.
A recent review discussing herbal and alternative medicines in influenza treatment details evidence suggesting that N-acetylcysteine, elderberry, or a combination of Eleutherococcus senticosus and Andrographis paniculata may help to shorten the course of influenza infection. The article cites more limited evidence including animal or in vitro studies to suggest possible benefit from vitamin C, DHEA, high lactoferrin whey protein, Echinacea spp., Panax quinquefolium, Larix occidentalis arabinogalactans, elenolic acid (a constituent of olive leaf extract), Astragalus membranaceus, and Isatis tinctoria or Isatis indigotica. Another review assessed the quality of evidence for alternative influenza treatments, it concluded that there was "no compelling evidence" that any of these treatments were effective and that the available data on these products is particularly weak, with trials in this area suffering from many shortcomings, such as being small and poorly-designed and not testing for adverse effects.
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