Anti-Influenza Drugs
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Direct answer
Anti-influenza drugs comprise the neuraminidase inhibitors oseltamivir, zanamivir and peramivir, which block viral release from infected cells, and the cap-endonuclease inhibitor baloxavir. Oseltamivir started within forty-eight hours of symptom onset shortens illness by about a day and is used for treatment and prophylaxis including in pregnancy and severe disease. The old M2-blockers amantadine and rimantadine are obsolete because circulating strains are resistant.
What you must remember
- Neuraminidase on the viral surface cleaves sialic acid residues so progeny virions can exit the cell and spread; inhibiting it confines virus locally — active against both influenza A and B.
- Oseltamivir is an oral sialic-acid-transition-state prodrug, given twice daily for five days; vomiting and nausea are the main adverse effects, and neuropsychiatric events in children have been reported.
- Zanamivir is an inhaled powder with identical mechanism — avoid in asthma or chronic obstructive pulmonary disease because of bronchospasm; peramivir is the intravenous member for severe illness.
- The forty-eight-hour rule: benefit in uncomplicated illness is greatest when treatment begins within two days of onset, but severely ill, hospitalised or high-risk patients should be treated regardless of delay.
- Baloxavir marboxil, a single-dose oral agent, inhibits the cap-dependent endonuclease of the viral polymerase complex, blocking mRNA priming — a mechanism entirely distinct from neuraminidase blockade.
- Amantadine and rimantadine block the M2 ion channel of influenza A only, preventing uncoating; universal resistance and their place in Parkinsonism therapy — amantadine releases dopamine — make them exam history; toxicity includes insomnia, confusion, livedo reticularis and ankle oedema.
- Prophylaxis: post-exposure oseltamivir or zanamivir for outbreaks in closed settings and protection of high-risk contacts, alongside annual vaccination as the primary strategy.
Common confusion
Candidates mix up the two attack points: neuraminidase inhibitors stop the release of new virions, whereas M2 blockers stop the entry-uncoating step — and baloxavir stops cap-snatching transcription. Equally confused are zanamivir and oseltamivir; the inhaled drug endangers asthmatics while the oral prodrug causes vomiting, a distinction examiners enjoy.
Exam-focused takeaway
Theory answers should classify by mechanism, stress the forty-eight-hour window, and mention pregnancy and severe-disease exceptions to that window. Viva examiners ask why amantadine failed and what balances antiviral chemoprophylaxis against vaccination. MCQs test neuraminidase function, oseltamivir for avian and pandemic influenza, zanamivir bronchospasm, baloxavir mechanism and amantadine in Parkinsonism.
Frequently asked questions
How do neuraminidase inhibitors work?
They prevent viral neuraminidase from cleaving sialic-acid receptors, so newly formed virions clump on the infected cell surface and cannot spread.
Why must oseltamivir be started within forty-eight hours?
Viral replication peaks in the first days of illness, so antiviral benefit wanes sharply afterwards — except in severe or high-risk patients, where treatment is still given.
Which anti-influenza drug is unsafe in asthma?
Inhaled zanamivir, which can provoke bronchospasm; oral oseltamivir is preferred in reactive airway disease.
What is baloxavir and how does it differ?
A single-dose oral inhibitor of the viral cap-dependent endonuclease, blocking the cap-snatching step of viral mRNA synthesis rather than virion release.
Why is amantadine no longer used for influenza?
Circulating influenza A strains carry M2 mutations conferring universal resistance; its surviving use is as an anti-Parkinsonian dopaminergic agent.