Antiviral Medicines for Flu and COVID-19: Types, Uses and Safety Guide

Every autumn and winter, respiratory viruses sweep through homes, schools and workplaces. Influenza, COVID-19, respiratory syncytial virus (RSV) and common cold viruses all cause similar symptoms of cough, fever, aches and fatigue, which makes it hard to know what you have and what to do. Antibiotics do nothing against viruses, but a separate group of medicines, called antivirals, can shorten some infections and reduce the risk of severe illness in people who are vulnerable. This guide explains how antiviral medicines work, the main treatments for flu and COVID-19 (such as oseltamivir, nirmatrelvir with ritonavir and molnupiravir), who benefits most, the all-important timing, the drug interactions to watch and the self-care steps that matter just as much as any tablet.

How Viruses and Antivirals Work

Viruses are tiny particles that cannot reproduce on their own. They enter your cells and hijack them to make copies. Antiviral medicines interrupt this process at a specific step, for example by stopping the virus from entering cells, copying its genetic material, assembling new particles or escaping infected cells. Because viruses use our own cells to multiply, designing safe antivirals is difficult, and most work against one virus or a small family of viruses. That is why a flu antiviral does nothing for COVID-19, and the reverse.

Flu or COVID-19 or a Cold? Telling Them Apart

Symptoms overlap so much that a rapid test is the only reliable way to tell flu from COVID-19. Testing early matters, because antivirals only work well when started within a narrow window.

Antivirals for Influenza

Oseltamivir

Oseltamivir is a neuraminidase inhibitor, which stops newly made flu viruses from leaving infected cells and spreading to others. It is effective against influenza A and B. Taken within 48 hours of symptom onset, it reduces the duration of illness by about a day and may lower the risk of complications such as pneumonia, particularly in higher-risk groups. A typical adult treatment course is twice daily for five days, with a lower dose once daily sometimes used for prevention in close contacts. Our store lists Oseltamivir. Common side effects are nausea and vomiting, which are reduced by taking it with food. Rarely, confusion or unusual behaviour has been reported, especially in children, so watch for these and seek advice if they occur.

Other flu antivirals

Baloxavir is a single-dose treatment that blocks viral replication. Zanamivir is inhaled and peramivir is given intravenously in hospital. Your clinician chooses depending on your age, health and local guidelines.

Antivirals for COVID-19

Nirmatrelvir with ritonavir

This combination (the brand Paxlovid is the best known) contains nirmatrelvir, which blocks a viral enzyme the coronavirus needs to multiply, and a small dose of ritonavir, which slows the breakdown of nirmatrelvir so it stays active longer. In high-risk adults, starting treatment within five days of symptom onset markedly reduced hospitalisation and death in clinical trials. The course lasts five days. See the generic nirmatrelvir/ritonavir product page.

Molnupiravir

Molnupiravir introduces errors into the virus’s genetic code as it copies itself, which stops it from multiplying effectively. It is generally considered less effective than nirmatrelvir with ritonavir, and some later large trials showed a quicker recovery but no clear reduction in hospital admission in vaccinated populations. It is used when other options are unsuitable. It is not recommended in pregnancy, and contraception advice applies during and after treatment. View the Molnupiravir product page.

Who Benefits Most From Antivirals?

Most healthy adults recover from flu or COVID-19 without antivirals. Treatment is prioritised for people at higher risk of severe disease, including:

• Adults aged 65 and over
• Pregnant women
• People with chronic lung, heart, kidney or liver disease, or diabetes
• People with weakened immune systems, including those taking immunosuppressants or chemotherapy
• People with severe obesity
• Residents of care homes

Your doctor weighs the benefit against the risks, drug interactions and kidney or liver function.

Timing Is Everything

Both flu and COVID-19 antivirals work by slowing viral multiplication, which peaks early in the illness. Waiting until you feel very unwell often means the window has closed. For oseltamivir, aim to start within 48 hours of symptoms, and for nirmatrelvir with ritonavir and molnupiravir, within five days. If you are in a higher-risk group, test promptly and contact your doctor on the same day you test positive or develop flu symptoms.

Drug Interactions: The Big Safety Issue With Ritonavir

Ritonavir strongly inhibits an enzyme called CYP3A4 that clears many medicines from the body. This can cause dangerous rises in the levels of other drugs. Interactions can affect:

• Statins such as simvastatin and lovastatin, which can cause muscle damage
• Blood thinners such as rivaroxaban and warfarin
• Heart rhythm medicines such as amiodarone
• Some sedatives and sleeping tablets, which may increase drowsiness. If you take sleeping medicines such as zopiclone, speak to a doctor first, because CYP3A4 inhibitors raise their levels. See insomnia medicines and how they work for background.
• Anticonvulsants, immunosuppressants and some cancer drugs
• St John’s wort and certain other herbal products, which are not allowed with ritonavir

Always give your doctor or pharmacist a full list of medicines and supplements before starting a COVID-19 antiviral.

Side Effects

• Oseltamivir: nausea, vomiting, headache, abdominal pain.
• Nirmatrelvir with ritonavir: altered taste, diarrhoea, nausea, muscle aches and raised blood pressure.
• Molnupiravir: diarrhoea, nausea, dizziness and headache.
• Seek urgent help for: allergic reactions with rash or swelling, severe shortness of breath, chest pain, confusion or fainting.

Symptom Relief and Self-Care

Whether or not you take an antiviral, the basics speed recovery and ease symptoms. Rest as much as possible, drink fluids to prevent dehydration, and use paracetamol or ibuprofen (if suitable for you) for fever and aches. Honey, warm drinks and saline nasal rinses can soothe coughs and congestion. Stay home while infectious to avoid passing the illness on, wear a mask around vulnerable people and ventilate rooms.

Sleep, Immunity and Recovery

Sleep is when your immune system does much of its work. People who sleep less than six hours are measurably more likely to catch viral infections, and poor sleep weakens vaccine responses. Illness itself disrupts sleep through coughing, fever and congestion, and pandemic stress made insomnia more common. Prop your head up with extra pillows to ease congestion, keep the room cool and avoid screens before bed. Read how sleep improves your immune system, sleep hygiene tips and home remedies for insomnia. For related guides, see antibiotic tablets explained.

Prevention Beats Treatment

Annual flu vaccination and up-to-date COVID-19 boosters remain the most effective way to avoid severe illness. Good hand hygiene, staying home when unwell, ventilation, adequate sleep, regular exercise and a balanced diet all lower your risk. Antivirals are a backup for those who still get sick, not a replacement for vaccination.

When to Seek Urgent Medical Help

Call emergency services or seek urgent care for difficulty breathing, persistent chest pain or pressure, blue lips or face, confusion, inability to stay awake, signs of dehydration, a fever that returns after improving or any rapid worsening, especially in older adults, pregnant women, young children and people with chronic illness.

Frequently Asked Questions

Do antivirals cure the flu or COVID-19?

They do not cure instantly, but they can shorten illness and reduce complications, especially when started early in higher-risk people.

Can I take an antiviral as prevention?

Oseltamivir can be used for post-exposure prevention in specific situations, but vaccination is the main preventive tool.

Can COVID-19 symptoms come back after Paxlovid?

A small proportion of people experience rebound symptoms or a positive test a few days after finishing treatment. Isolate again and contact your doctor if symptoms are severe.

Are antivirals safe in pregnancy?

Oseltamivir is generally recommended in pregnancy when flu is suspected. Molnupiravir is not recommended. Discuss options with your doctor.

Can I drink alcohol while taking antivirals?

Alcohol is best avoided while you are unwell, because it dehydrates you and can worsen nausea and sleep.

Other Antivirals You May Hear About

Antivirals are not limited to respiratory infections. Aciclovir and valaciclovir treat herpes simplex and shingles by blocking viral DNA copying, and they work best when started at the first tingle or blister. Combination antiretroviral therapy controls HIV so effectively that people with an undetectable viral load cannot pass the virus on sexually. Modern direct-acting antivirals cure the great majority of hepatitis C infections in eight to twelve weeks, and tenofovir or entecavir can suppress hepatitis B long-term. Each of these needs specialist assessment, testing and monitoring, so they are not suited to self-treatment.

Antiviral Resistance: A Growing Concern

Just as bacteria can become resistant to antibiotics, viruses can mutate to resist antivirals. Influenza viruses have developed resistance to older drugs such as amantadine, which is why they are no longer recommended, and resistance to oseltamivir is monitored every season by health authorities. Using antivirals only when they are likely to help, completing the prescribed course and avoiding leftover tablets slow this process. It is another reason why guidance prioritises people at high risk rather than treating every mild case.

Looking After Children and Older Adults

Children

Children with flu or COVID-19 usually recover at home, but they can deteriorate quickly. Watch for fast or laboured breathing, drawing-in of the chest, dehydration (no urine for eight hours, dry mouth, no tears), unusual sleepiness or irritability, or a rash that does not fade when pressed. Never give aspirin to children or teenagers with viral illness, because of the risk of Reye’s syndrome. Oseltamivir is available in liquid form with weight-based doses, so adult tablets should not be used.

Older adults

Older adults may not develop a high fever, and confusion or unusual tiredness can be the first sign of infection. They have a higher risk of pneumonia and dehydration, so early testing and contact with a doctor are important. Review all medicines before starting antivirals, since many older people take several drugs that may interact.

Common Myths About Antivirals and Viral Illness

• “Antibiotics will help my flu.” They have no effect on viruses unless a bacterial infection develops as a complication.
• “Antivirals replace vaccination.” Vaccines prevent severe disease, while antivirals treat infection after it starts.
• “If I feel fine on day three, I can stop early.” Completing the full course reduces relapse and resistance.
• “Vitamin C or zinc will cure it.” Evidence suggests they may slightly shorten colds in some people, but they do not treat flu or COVID-19.
• “Natural immunity from past infection is enough.” Immunity wanes and viruses change, so vaccination remains important.

Build a Simple Home Kit Before You Need It

• A thermometer and a few rapid COVID-19 or flu tests within their expiry date
• Paracetamol and, if appropriate, ibuprofen, with the dosing instructions
• Oral rehydration sachets, honey, saline spray and lozenges
• A pulse oximeter if your doctor recommends it for high-risk conditions
• A written list of your medicines to show a doctor or pharmacist quickly
• Contact numbers for your doctor, local pharmacy and out-of-hours service

Questions to Ask Your Doctor

• Am I in a group that would benefit from an antiviral?
• Which antiviral is safest with my other medicines and health conditions?
• How soon must I start, and how should I take it?
• What should I do if my symptoms worsen or return after treatment?
• When can I return to work or school safely?

Final Thoughts

Antiviral medicines are valuable tools, but they work only for the right virus, in the right person, at the right time. Test early, check your medicines for interactions, look after your sleep and hydration, and speak to a healthcare professional promptly if you are in a higher-risk group.

Disclaimer: This article is for general information only and is not medical advice. Always consult a doctor or pharmacist before taking any medicine.