In the present study we evaluated drug-drug interaction potential of ambroxol and cetirizine mediated by inhibition of CYP isoforms including CYP1A2, CYP2A6, CYP2B6, CYP2C9, CYP2C19, CYP2D6, CYP2E1 and CYP3A4 using pooled human liver microsomes (HLMs). As measured by liquid chromatography-electrospray ionization tandem mass spectrometry, cetirizine and ambroxol inhibited significantly CYP2E1 but the maximal inhibition was approximately 36% at 10 ${\mu}M$ cetirizine and 28% at 3 ${\mu}M$ ambroxol. In addition, CYP2D6 activity was decreased to approximately 83% of control activity in pooled HLM incubated with 3 ${\mu}M$ ambroxol. Activities of CYP1A2, CYP2A6, CYP2B6, CYP2C9, CYP2C19, and CYP3A4 were not significantly inhibited by cetirizine and ambroxol. Considering their maximal plasma concentration in human ($C_{max}$ of cetirizine is approximately 0.67 ${\mu}M$ and $C_{max}$ of ambroxol is 0.044 ${\mu}M$), these two drugs have very low possibility in drug-drug interaction by CYP inhibition in clinical situations.