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Understanding the science behind PAXLOVIDThe first approved protease inhibitor designed to treat COVID-191,2,6PAXLOVID in action 
PAXLOVID's active pharmaceutical ingredient is nirmatrelvir, an oral protease inhibitor specifically designed by Pfizer to inhibit the main protease (M[pro]), also known as 3CL protease, and to help halt SARS-CoV-2 replication by preventing proteolysis from occurring. 1,3
  • PAXLOVID also contains ritonavir, a CYP3A inhibitor that is used as a pharmacokinetic enhancer. 1
How PAXLOVID works1. Proteolysis​​​​​​​

In the SARS-CoV-2 life cycle, proteolysis involves the cleavage of viral polyprotein chains by proteases to release the nonstructural proteins that are essential for viral replication.3

2. Mpro

Due to its essential function in viral replication, mutations that occur to Mpro can be lethal to the virus.4

3. Nirmatrelvir

Inhibition of Mpro by nirmatrelvir prevents proteolysis, helping to halt the viral replication cycle.1,3

The role of ritonavir1

Ritonavir is administered with nirmatrelvir as a pharmacokinetic (PK) enhancer, resulting in higher systemic concentrations of
nirmatrelvir and helping to support twice-daily dosing.1,2

  • Ritonavir has an established safety profile and is used in combination with other antivirals as a PK enhancer1,2,5
EXPLORE MOREDosing

Learn about the oral treatment schedule, dose adjustments, and drug interactions.

 Dosing information Loading Dose adjustments Loading Drug interactions LoadingReferences:
  1. PAXLOVID Summary of Product Characteristics.
  2. Owen DR, Allerton CMN, Anderson AS, et al. An oral SARS-CoV-2 Mpro inhibitor clinical candidate for the treatment of COVID-19. Science. 2021;374(6575):1586-1593.
  3. Senger MR, Evangelista TCS, Dantas RF, et al. COVID-19: molecular targets, drug repurposing and new avenues for drug discovery. Mem Inst Oswaldo Cruz. 2020;115:e200254.
  4. Goyal B, Goyal D. Targeting the dimerization of the main protease of coronavirus: a potential broad-spectrum therapeutic strategy. ACS Comb Sci. 2020;22(6):297-305.
  5. NORVIR Summary of Product Characteristics. https://www.medicines.ie/medicines/norvir-100mg-film-coated-tablets-33125/spc (Accessed July 2025). 
  6. Business Wire. Pfizer’s PAXLOVID™ Receives FDA Approval for Adult Patients at High Risk of Progression to Severe COVID-19. https://www.businesswire.com/news/home/20230525005573/en/ (Accessed July 2025).

▼ This medicinal product is subject to additional monitoring. This will allow quick identification of new safety information. Healthcare professionals are asked to report any suspected adverse reactions. See section 4.8 of the SmPC for how to report adverse reactions.

Legal Category: S1A
Further information is available upon request
PP-C1D-IRL-0090. August 2025

Adverse events should be reported.

If you wish to make a medical information inquiry or report an adverse event please contact Pfizer on 1800 633 363 
or email Pfizer at [email protected] or visit www.PfizerMedicalInformation.ie

Report an adverse event to your national reporting system (HPRA Pharmacovigilance)

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