↓ voriconazole High doses of ketoconazole or itraconazole (> 200 mg/day) are not recommended.

Coadministration of voriconazole and ritonavir doses of 400 mg every 12 hours or greater is contraindicated. Coadministration of voriconazole and ritonavir 100 mg should be avoided, unless an assessment of the benefit/risk to the patient justifies the use of voriconazole.
 
Anti-gout:
colchicine ↑ colchicine Patients with renal or hepatic impairment should not be given colchicine with ritonavir.

Treatment of gout flares-co-administration of colchicine in patients on ritonavir:

0.6 mg (1 tablet) x 1 dose, followed by 0.3 mg (half tablet) 1 hour later. Dose to be repeated no earlier than 3 days.

Prophylaxis of gout flares-co-administration of colchicine in patients on ritonavir:

If the original colchicine regimen was 0.6 mg twice a day, the regimen should be adjusted to 0.3 mg once a day.

If the original colchicine regimen was 0.6 mg once a day, the regimen should be adjusted to 0.3 mg once every other day.

Treatment of familial Mediterranean fever (FMF)-co-administration of colchicine in patients on ritonavir:

Maximum daily dose of 0.6 mg (may be given as 0.3 mg twice a day). 
Anti-infective:
clarithromycin ↑ clarithromycin For patients with renal impairment the following dosage adjustments should be considered:

For patients with CLCR 30 to 60 mL/min the dose of clarithromycin should be reduced by 50%.
For patients with CLCR < 30 mL/min the dose of clarithromycin should be decreased by 75%.

No dose adjustment for patients with normal renal function is necessary.
Antimycobacterial:
rifabutin ↑ rifabutin and rifabutin metabolite Dosage reduction of rifabutin by at least three-quarters of the usual dose of 300 mg/day is recommended (e.g., 150mg every other day or three times a week). Further dosage reduction may be necessary.
Antimycobacterial:
rifampin ↓ ritonavir May lead to loss of virologic response. Alternate antimycobacterial agents such as rifabutin should be considered (see Antimycobacterial: rifabutin, for dose reduction recommendations).
Antiparasitic:
atovaquone ↓ atovaquone Clinical significance is unknown; however, increase in atovaquone dose may be needed.
Antiparasitic:
quinine ↑ quinine A dose decrease of quinine may be needed when co-administered with ritonavir.
β-Blockers:
metoprolol, timolol ↑ Beta-Blockers Caution is warranted and clinical monitoring of patients is recommended. A dose decrease may be needed for these drugs when co-administered with ritonavir.
Bronchodilator:
theophylline ↓ theophylline Increased dosage of theophylline may be required; therapeutic monitoring should be considered.
Calcium channel blockers:
diltiazem, nifedipine, verapamil ↑ calcium channel blockers Caution is warranted and clinical monitoring of patients is recommended. A dose decrease may be needed for these drugs when co-administered with ritonavir.
Digox