Hepatic arteriovenous malformations (HAVMs) in hereditary hemorrhagic telangiectasia (HHT) have long been considered to have scarce clinical significance in most cases. Nevertheless, data are lacking regarding the influence of HAVMs on the liver first-pass effect on drugs in HHT patients. To gain insight into the effect of HAVMs on hepatic drug clearance by means of two specific (13)C-labeled probes, namely the (13)C-methacetin and (13)C-aminopyrine, 46 HHT patients and 44-matched healthy controls were enrolled. The liver first-pass effect was studied by the (13)C-based breath test using methacetin and aminopyrine. The methacetin breath test showed statistically significant reduced metabolism rates (p < 0.0001) in HHT when compared with controls, both in patients with and without CT-detectable HAVMs, and when expressed both as cumulative (13)C-percentage dose per hour and as (13)C-percentage peak after 15 min. In contrast, no significant difference was found between HHT and controls regarding aminopyrin metabolism rates. In HHT, (13)C%-methacetin breath test values are significantly lower than those found in normal subjects, probably due to the effect of hepatic shunts. A reduced perfusion and an impaired hepatic metabolism might affect hepatic drug clearance in HHT. Therefore, an appropriate dosage adjustments should be considered when high-hepatic-metabolism drugs are administered to HHT patients.