The interaction between ritonavir (600 mg, twice daily) and cetirizine (10 mg, once daily) was investigated in 16 HIV-negative male subjects. Coadministration of cetirizine resulted in moderate decreases of 11% in ritonavir AUC and 13% in Cmax. These were not statistically significant and were considered not to be clinically relevant. Coadministration of ritonavir resulted in a 42% increase in cetirizine AUC and a 9% increase in Cmax. Half life increased by 53% and clearance decreased by 29%. The increased extent of cetirizine exposure seen in healthy volunteers with ritonavir was in the same range as previously observed in elderly subjects or those with mild renal impairment. These results, obtained with ritonavir 600 mg twice daily, are probably an overestimate compared with those that might be observed with a 100 mg twice daily dose.
Evaluation of pharmacokinetic interaction between cetirizine and ritonavir, an HIV-1 protease inhibitor, in healthy male volunteers. Peytavin G, Gautran C, Otoul C, et al. Eur J Clin Pharmacol. 2005 Jun;61(4):267-73.

The electrocardiographic effect of cetirizine was studied in 10 HIV+ subjects who were receiving PIs (details not given). No patient had significant electrocardiographic changes during cetirizine administration. The authors concluded that cetirizine is a safe antihistamine drug in patients treated with PIs.
Cetirizine is a safe antihistamine drug in HIV-infected patients under protease inhibitor treatment. Estrada JAL, Flores AD, Hernandez GC & Urdez HE. 13th International AIDS Conference, Durban, July 2000, abstract WePeB 4229.