The effect of milk thistle (450 mg three times daily) on the pharmacokinetics of indinavir (800 mg three times daily) was investigated in HIV-negative subjects. Indinavir AUC and Cmax in the control group (n=8) were 21.7 µg/ml.h and 9.28 µg/ml, respectively. Indinavir concentrations were not significantly reduced in the milk thistle group (n=8) with AUC and Cmax values of 19.1 µg/ml.h and 9.03 µg/ml, respectively.
Milk thistle and indinavir: a randomized controlled pharmacokinetics study and meta-analysis. Mills E, Wilson K, Clarke M, et al. Eur J Clin Pharmacol, 2005, 61(1):1-7.

Indinavir pharmacokinetics were investigated in 10 healthy volunteers both alone (800 mg three times daily) and after administration of milk thistle (160 mg three daily for 12 days). Milk thistle had no apparent effect on indinavir pharmacokinetics. After administration of milk thistle, geometric mean indinavir AUC decreased from 20.7 (15.3-29.2) mg/L.h to 19.4 (15.8-23.6)mg/L.h and Cmin decreased from 0.340 (0.232-0.497) mg/L to 0.232 (0.129-0.419) mg/L. Concomitant administration of milk thistle at this dose would not be expected to alter indinavir AUC, Cmax and Cmin to clinically significant extents.
Coadministration of milk thistle and indinavir in healthy subjects. DiCenzo R, Shelton M, Jordan K, et al. Pharmacother, 2003, 23: 866-870.

The effect of milk thistle (175 mg three times daily for 3 weeks) on 4 doses of indinavir (800 mg every 8 h) was evaluated in healthy volunteers. Milk thistle did not significantly alter the overall exposure of indinavir (9% decrease in AUC), however, the mean trough indinavir concentration was significantly decreased by 25%.
The effect of milk thistle on the pharmacokinetics of indinavir in healthy volunteers. Piscitelli SC, Formentini E, Burstein AH, et al., 2001, Pharmacother, 2002, 22:551-556

In an in vitro study there was the potential for milk thistle to inhibit the metabolism of CYP3A4 and UGT1A substrates in human liver. The study indicated that use of milk thistle with drugs that are conjugated by UGT1A may result in altered clearance of certain drugs. There is the potential for drug–herb interactions with silymarin. Further studies are needed.
Milk thistle, an herbal supplement, decreases the activity of CYP3A4 and uridine diphosphoglucuronosyl transferase in human hepatocyte cultures. Venkataramanan R, Ramachandran V, Komoroski BJ, et al. Drug Metab Dis, 2000, 28:1270–3.