There are mainly four approved drugs used to treat or prevent influenza – amantadine, oseltamivir, rimantadine and zanamivir. Rimantadine and amantadine are effective only against type A influenza. Zanamivir and oseltamivir inhibit both influenza A and B viruses.

a) Adamantane Deriatives (Amantadine/Rimantadine)

How they work

These two drugs are chemically related, and act only against Influenza type A viruses. The inhibit the activity of the virus M2 protein , which forms a channel in the virus membrane. This protein is crucial in enabling the virus to enter its target cell, thus the result of this would be disabling the virus.

Side Effects

Amantadine and rimantadine may cause CNS (Central Nervous System) and gastrointestinal adverse side effects in healthy young adults. However, incidence of CNS side effects like anxiety, insomnia, difficulty in concentrating, lightheadedness and nervousness is apparently reported to be higher with amantadine than with rimantadine. This could be due to the fact that rimantadine has been marketed for a shorter period, and thus has not been evaluated and tested as many times. Gastrointestinal side effects like nausea and anorexia can occur in a small percentage of persons taking either drug.

Usually, side effects of amantadine and rimantadine are mild and cease soon after administration of the the drug is stopped, and can diminish or disappear after the first week even with continued drug ingestion. Serious side effects, like marked behavioral changes, delirium, hallucinations, agitation, and seizures, have been observed however. These severe side effects have been associated with high plasma drug concentrations and are usually observed most often among old people or those who already have underlying renal insufficiency, seizure disorders, or certain psychiatric disorders. Also, when an overdosage of these drugs is administered, CNS, renal, respiratory, and cardiac toxicity can occur.

b) Neuraminidase Inhibitors (Oseltamivir/Zanamivir)

How they work

The surfaces of influenza viruses are dotted with neuraminidase glycoproteins. The enzyme Neuraminidase is needed for the viruses to break free from an infected cell after replication, setting free new viruses that can infect other cells and spread infection. Neuraminidase inhibitors block the enzyme's activity, preventing new virus particles from being released and thereby limiting the spread of infection. Oseltamivir and Zanamivir are the two approved neuraminidase inhibitors. Oseltamivir (commonly known as Tamiflu) is orally administered in tablet form, while Zanamivir (commonly known as Relenza) is administered via an inhaler.

Side Effects

Few serious central nervous system (CNS) adverse effects have been reported for the neuraminidase inhibitor drugs. Nausea and vomiting have been observed in patients after Oseltamivir had been administered to them.

Nausea, diarrhea, dizziness, headache, and cough have also been reported during zanamivir treatment, but these were also seen when patients were given inhaled powdered placebo (fake) drug. A logical hypothesis would be that such symtoms are probably caused by the inhalation process rather than the drug itself. Thus, Zanamivir is not recommended for use in patients with respiratory diseases due to its property of having to be inhaled.

c) Comparing the Drugs

In general, the Adamantane deriative drugs seem to be as clinically effective as the neuraminidase inhibitor drugs. It is noted however that there are fewer reports of the neuraminidase inhibitors causing serious side effects than compared to Amantadine and Rimantadine, though the frequency or severity of side effects between the two drugs have not been directly compared. R esistance towards the a damantane deriative drugs also seem to arise more rapidly compared to the neuraminidase inhibitors.

Tamiflu ( Oseltamivir) is also preferred between the two neuraminidase inhibitors, mainly because of the fact that, as mentioned above, Relanza ( Zanamivir) has to be inhaled and would not be practical if the patient has breathing problems or respiratory diseases arising from the infection itself. Hence most experts currently agree that Tamiflu is the best of the four available drugs, however other drugs should be considered if the virus seems to develop a resistance towards Tamiflu, which is already happening to some extent.