To compare the supply of molnupiravir and nirmatrelvir/ritonavir in relation to patient characteristics and other co-prescribed medicines and to estimate the number of patients without contraindications to nirmatrelvir/ritonavir who were treated with molnupiravir.

Retrospective observational study of patients identified in the Pharmaceutical Benefits Scheme (PBS) 10 % sample dataset who were supplied with either molnupiravir or nirmatrelvir/ritonavir between May and December 2022. We supplemented the PBS dataset with aggregated counts from published literature to determine prevalence of clinical contraindications to nirmatrelvir/ritonavir.

We used multivariable Poisson regression to estimate risk ratios (RR) of receiving nirmatrelvir/ritonavir over molnupiravir.

We identified 54,550 patients who received either nirmatrelvir/ritonavir (26.8 %) or molnupiravir (73.2 %). Their average age was 71.6 (SD = 13.4) years and 57.1 % were female. Patients were less likely to receive nirmatrelvir/ritonavir with increasing age (RR = 0.50; 95 % CI: 0.48–0.53; for ages 85 + compared to < 65 years) or who had received medicines contraindicated for use with nirmatrelvir/ritonavir (RR = 0.66; 95 % CI: 0.64–0.68). During the study period, we estimated that between 28.4 % and 45.4 % of patients aged ≥ 65 years had received molnupiravir in the absence of pharmacological and clinical contraindications to nirmatrelvir/ritonavir.

Many prescriptions were written for molnupiravir where there were no contraindications to nirmatrelvir/ritonavir. The benefits that followed from prompt government action in approving and obtaining nirmatrelvir/ritonavir were therefore likely to be less than they could potentially have been. Governments should consider investing in quality improvement systems to ensure the best outcomes in terms of efficacy and safety.