Coadministration of ritonavir (300 mg twice daily for three days) and a single IV dose of digoxin (0.5 mg) increased digoxin AUC by 86%. Coadministration of ritonavir (200 mg twice daily for 13 days) and a single ORAL dose of digoxin (0.4 mg) increased digoxin AUC by 22% and had no effect on Cmax. Particular caution should be used when prescribing ritonavir in patients taking digoxin since co-administration of ritonavir with digoxin is expected to increase digoxin levels. The increased digoxin levels may lessen over time. In patients who are already taking digoxin when ritonavir is introduced, the digoxin dose should be reduced to one-half of the patients' normal dose and patients need to be followed more closely than usual for several weeks after initiating co-administration of ritonavir and digoxin. In patients who are already taking ritonavir when digoxin is introduced, digoxin should be introduced more gradually than usual. Digoxin levels should be monitored more intensively than usual during this period, with dose adjustments made, as necessary, based on clinical, electrocardiographic and digoxin level findings.
Norvir Summary of Product Characteristics, AbbVie Ltd, September 2016.

Concomitant administration of ritonavir with digoxin may increase digoxin levels. Caution should be exercised when coadministering ritonavir with digoxin, with appropriate monitoring of serum digoxin levels. The impact on the PR interval of co-administration of ritonavir with other drugs that prolong the PR interval (including digoxin) has not been evaluated. As a result, co-administration of ritonavir with these drugs should be undertaken with caution, particularly with those drugs metabolized by CYP3A. Clinical monitoring is recommended.
Norvir Prescribing Information, AbbVie Inc, December 2016.

The pharmacokinetics of digoxin were determined in 12 healthy volunteers following a single dose of digoxin (0.4 mg orally) before and after 14 days of ritonavir (200 mg twice daily). Ritonavir increased digoxin AUC(0-72) from 26.20 ± 8.67 to 31.96 ± 11.24 ng.h/mL and AUC(0-8) from 6.25 ± 1.8 to 8.04 ± 2.22 ng.h/mL. Digoxin oral clearance decreased from 149 ± 101 to 105 ± 57 mL/h.kg. Other digoxin pharmacokinetics, including renal clearance, were unaffected by ritonavir. Overall, 75% (9/12) of subjects had higher concentrations of digoxin after ritonavir administration. The most likely mechanism for this interaction is ritonavir-associated inhibition of P-gp.
Ritonavir decreases the nonrenal clearance of digoxin in healthy volunteers with known MDR1 genotypes. Penzak SR, Shen JM, Alfaro RM, et al. Ther Drug Monit, 2004, 26(3): 322-330.

Coadministration of ritonavir (300 mg twice daily for 11 days) and digoxin (0.5 mg IV single dose was studied in 12 HIV- subjects. Ritonavir significantly increased digoxin AUC by 86% and volume of distribution by 77%; nonrenal and renal digoxin clearance were decreased by 48% and 35%, respectively. Digoxin terminal half-life in plasma increased by 156%.
Substantial pharmacokinetic interaction between digoxin and ritonavir in healthy volunteers. Ding R, Tayrouz Y, Riedel KD, et al. Clin Pharmacol Ther, 2004, 76(1): 73-84.