Coadministration has not been studied but can be considered. Anastrozole is mainly oxidized to hydroxyanastrozole by CYP3A4 (major) and glucuronidated by UGT1A4. Nirmatrelvir/ritonavir strongly inhibits CYP3A4 and is expected to increase the concentrations of anastrozole. The clinical relevance of this interaction is unknown. Monitoring for anastrozole toxicity may be required. Ritonavir may also induce glucuronidation. However, given that induction reaches maximal effect after several days and the short duration of nirmatrelvir/ritonavir treatment, no effect on anastrozole due to induction of UGTs anticipated.