The antiviral in Paxlovid
Nirmatrelvir
An antiviral for early COVID-19 in higher-risk people, always taken with ritonavir as Paxlovid.
What is Nirmatrelvir?
Nirmatrelvir is the antiviral medicine inside Paxlovid, a treatment for COVID-19. It is always taken together with a second medicine, ritonavir, which 'boosts' it so it works well, and the two are supplied as one combined treatment. It is used early in the illness for people at higher risk of becoming seriously unwell, as a short course over a few days. Because of the ritonavir part, it has major drug interactions, and it is not suitable for people with severe kidney or liver problems.
Education and reference only. This is a plain-language guide to Nirmatrelvir — it deliberately contains no doses. Doses depend on the person, the brand and the reason for treatment, and belong with your prescriber. Always check the BNF, the product labelling (SmPC) and follow medical advice.
What it is
Nirmatrelvir is an antiviral that targets COVID-19. It is never used alone: it comes combined with ritonavir as Paxlovid, where the ritonavir keeps the nirmatrelvir at the right level in the body. It is intended for people who have early, mild-to-moderate COVID-19 and are at higher risk of becoming seriously ill, for example because of their age or certain health conditions. It is taken by mouth as a short course.
How it works
Nirmatrelvir blocks an enzyme the COVID-19 virus needs in order to copy itself, slowing the spread of the virus in the body so the illness is less likely to become severe. On its own it would be broken down too quickly, so it is paired with ritonavir, which blocks the liver enzyme that would clear it and keeps it working. This pairing is why it must be taken with ritonavir and why it carries ritonavir's wide range of drug interactions.
Company & origin
Originated / developed by: Pfizer.
An antiviral used in the UK for early COVID-19 in higher-risk people, always given together with ritonavir as Paxlovid.
Practical use
How to take Nirmatrelvir
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- Start it as soon as possible after symptoms begin and a positive COVID-19 test, as advised.
- Take the full short course, with both medicines together, exactly as prescribed.
- Give your prescriber and pharmacist a full list of all your medicines, supplements and herbal remedies first.
- Be ready to pause or adjust some of your usual medicines during the course, on professional advice.
- Tell your prescriber if you have kidney or liver problems, as it may not be suitable or may need a lower dose.
- Complete the course even if you feel better, and report if symptoms return afterwards.
Weighing it up
Advantages & disadvantages of Nirmatrelvir
Advantages
- Can reduce the risk of COVID-19 becoming severe in higher-risk people when started early.
- A short course taken by mouth at home, rather than treatment in hospital.
- Targets the virus directly to slow how fast it multiplies.
Disadvantages
- Has major drug interactions because of the ritonavir it is combined with.
- Not suitable in severe kidney or liver problems, and needs a lower dose in moderate kidney impairment.
- Only helpful early in the illness, and some people find symptoms return after the course.
Practical use
Good to know
Nirmatrelvir is only useful early in COVID-19, so it is started as soon as possible after symptoms begin and a positive test, as a short course. It is always taken with ritonavir as the combined treatment Paxlovid. Because of the ritonavir part, it has major drug interactions: some other medicines must be paused, adjusted or avoided while you take it, so your prescriber and pharmacist must see a full list of everything you use, including supplements and herbal remedies. It is not suitable for people with severe kidney or liver problems, and the dose is reduced in moderate kidney impairment. Because of the ritonavir part, hormonal contraception (such as the pill) can become less reliable during and shortly after the course, so an additional barrier method such as condoms is advised. Some people notice an altered or metallic taste. A small number find symptoms return after finishing the course.
Who should not take it / use with caution
- People with severe kidney or severe liver problems should not take it.
- Those taking certain other medicines that ritonavir can raise to dangerous levels must not combine them.
- Anyone who has had a serious allergic reaction to nirmatrelvir or ritonavir should not take it.
Monitoring
- A careful review of all other medicines, supplements and herbal remedies before starting.
- Checking kidney and liver function to confirm it is suitable and the dose is right.
- Watching for a return of symptoms after the course and for any troublesome effects.
Side effects
- An altered or metallic taste is common.
- Diarrhoea, nausea or stomach upset in some people.
- Headache or, less often, a return of symptoms after finishing the course.
Key interactions
- Has major interactions through the ritonavir part, which can raise levels of many other medicines.
- Some everyday medicines must be paused, adjusted or avoided during the short course.
- St John's wort and certain other products can interfere, so every medicine must be checked first.
Available as: Tablets taken by mouth (supplied with ritonavir as a combined pack).
Answers
Nirmatrelvir: frequently asked questions
What is nirmatrelvir used for?
It is the antiviral in Paxlovid, used early in COVID-19 for people at higher risk of becoming seriously unwell; it is always taken together with ritonavir as a short course.
Why is it taken with ritonavir?
On its own it would be broken down too quickly, so ritonavir is added to keep it at the right level; this pairing is also why it has many drug interactions.
When should I start it?
It only works early in the illness, so it is started as soon as possible after symptoms begin and a positive test, as advised by your prescriber.
Can anyone take it?
No. It is not suitable for people with severe kidney or liver problems, and a lower dose is used in moderate kidney impairment; your prescriber will check this.
Why must my other medicines be checked?
Because of the ritonavir part, some medicines must be paused, adjusted or avoided during the course, so your prescriber and pharmacist need a full list of everything you take.
Authoritative sources
- BNF
- NICE CKS
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