Seasonal Influenza

NoteDoses are oral and for adults unless otherwise stated. Visit the Health Products Regulatory Authority website for drug SPCs, dosage, contraindications, interactions, or IMF/BNF/BNFC/MIMS. See guidance on dosing in children for quick reference dosage/weight guide.

Letters indicating strength of evidence range from A+ (systematic review) to D (informal opinion). Statins can interact with some antibiotics and increase the risk of rhabdomyolysis. Amiodarone and drugs which prolong the QT interval can interact with many antibiotics. Many antibiotics increase the risk of bleeding with anticoagulants. Please refer to our Drug Interactions Table for further information.


  • When influenza A or B is circulating in the community, antiviral therapy should be considered for influenza-like illness in patients who are very ill or who are in recognised risk groups for severe influenza.
  • For otherwise healthy adults antivirals are not generally recommended.
  • More detailed (2015) information on treatment of or chemoprophylaxis for seasonal influenza, including in pregnant or immunosuppressed patients is available in short summary document by clicking here.
  • The editorial group recommend that these HPSC guidelines are consulted prior to making the final prescribing decision.

Key points from 2014-2015 HPSC Antiviral Treatment of Influenza Guidelines are as follows:

  1. Previously healthy people ( excluding pregnant women ) do not need antiviral treatment unless the clinician feels the patient is very ill or is at serious risk of developing complications from influenza. Symptomatic treatment is the preferred option.
  2. At risk population: prescribe oseltamivir (PO). Do not wait for laboratory confirmation. Treatment should be started as soon as possible, ideally within 48 hours of onset. There is evidence that treatment may reduce the risk of severe illness up to five days after onset.Treatment after 48 hours is an off-label use of oseltamivir and clinical judgement should be used.
  3. Severely immunosuppressed patients: prescribe oseltamivir (PO) Rapid emergence of oseltamivir resistance on treatment has been described in these patients and they should be monitored closely. Resistance to oseltamivir has been described in infections from influenza A (H1N1) pdm09 subtype but not in those from influenza A (H3) to date (source: NVRL). Clinicians may consider the use of zanamivir (inhaler) as first line therapy in immunosuppressed patients with suspected or confirmed influenza A (H1N1) pdm09 based on clinical judgement. In immunosuppressed patients, if no clinical improvement is seen within 5 days, patients should be tested for antiviral resistance.
  4. Suspected or confirmed oseltamivir resistant influenza in a patient who requires treatment: Zanamivir (inhaler) 10 mg (2 inhalations) bd up to ten days (off label duration). Treatment should be started as soon as possible and ideally within 36 hours of symptom onset.

The official detailed prescribing information recommended by the HPRA including the patient information leaflet for all formulations can be found here Tamiflu SmPCRelenza SmPC

Risk Groups

Defined Risk Groups for Antivirals

  • Children aged < 2 years
  • Pregnant women (including up to 2 weeks post partum) *
  • People aged 65 years and older
  • Severely obese people (BMI≥40) 
  • Children with any condition (e.g. cognitive dysfunction, spinal cord injury, seizure disorder or other neuromuscular disorder) that may compromise respiratory function, especially those attending special schools/day centres
  • Children with moderate to severe neurodevelopmental disorders such as cerebral palsy and intellectual disability
  • People with chronic respiratory disease including people on medication for asthma
  • People with chronic heart, kidney, liver or neurological disease
  • People with immunosuppression (weakened immune system whether due to disease or treatment)
  • People with Diabetes Mellitus
  • People with Haemoglobinopathies


See guidance on dosing in children for quick reference dosage/weight guide.

Table 1: Antiviral Treatment dosages and schedules for treatment

month 6
1-12 years: Dose according to weight below: Adults
≤ 15kg >15-23kg >23-40kg >40kg
Oseltamivir PO
(treatment course: 5 days)
dose bd
dose bd
dose bd
30mg bd 45mg bd 60mg bd 75mg bd 75mg bd
Zanamivir inhaled (treatment course: 5 days) Not licenced for children <5 years of age.
Adults and children ≥5 years: 10mg (2 inhalations) bd approximately 12 hours apart
10mg bd

Table 2: Antiviral dosage and schedules for chemoprophylaxis

Prophylaxis 0-1
month 8
1-12 years: Dose according to weight below Adults
(13 years and over)
≤ 15kg >15-23kg >23-40kg >40kg
Oseltamivir PO
(Prophylaxix course: 10 days)


30mg od 45mg od 60mg od 75mg od 75mg od
0-4 years ≥ 5 years (children and adults)
Zanamivir inhaled (Prophylaxis course: 5 days) N/A Not licenced for use 10mg (2 inhalations) once daily
The full HPSC Antiviral Treatment of Influenza Guidelines are available here.

*Advice of the influenza subgroup of the scientific advisory committee, HPSC on the use of antivirals in pregnancy, for patients with influenza

  • All pregnant women with influenza like illness should seek medical advice.
  • Pregnant women at less than 14 weeks gestation who have moderate to severe symptoms or who have a medical condition that puts them at high risk of complications from influenza are commenced on Oseltamivir treatment as soon as possible following diagnosis of influenza like illness.
  • All pregnant women at 14 to 42 weeks gestation, and women up to 2 weeks post partum, are commenced on Oseltamivir treatment as soon as possible following diagnosis of influenza like illness.
  • Evidence to date suggests that Oseltamivir or Zanamivir are safe to use at less than 14 weeks gestation, but like all other medications administered at less than 14 weeks gestation, should only be used when the benefit to the mother justifies the potential risk to the foetus. Therefore, pregnant women at less than 14 weeks gestation who have mild influenza like illness and no co‐morbidities may be managed symptomatically with paracetamol and fluids and observed closely for any deterioration. The clinical picture is not always clear‐cut initially, and hence the need for observation of the patient. Antivirals should be used if symptoms are severe; if there are other risk factors as well as pregnancy; or if based on a discussion between the mother and the clinician on the risks of infection, and the information available on the safety of antiviral use in pregnant women, the clinician judges it to be appropriate. All pregnant women at less than 14 weeks gestation (and those planning to conceive) should be advised to take folic acid.
  • Chemoprophylaxis is not routinely recommended for pregnant women. However, clinical judgement may be exercised in individual cases if the benefit outweighs the risk.

Patient Information

The HSE Health A-Z website provides patient information on many hundreds of conditions and treatments.

We recommend patients use the website developed by HSE/ICGP/IPU partnership for tips on how to get better from common infections without using antibiotics, what you can do for yourself or a loved one and when to seek help.  

Reviewed June 2016

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