Administration of high doses of acetaminophen (APAP) is known to cause drug-induced liver injury (DILI) in humans. Therefore, the detection or prediction of these side-effects at an early stage using appropriate biomarkers is the need of the hour. Micro RNA (miR)-122 is expected to be a novel biomarker for liver injury. However, more evidence is required in various alternate situations such as its use in combination as APAP is often used along with anticancer drugs. In the present study, we aimed to evaluate the functions of miR-122 as a biomarker for liver injury in comparison with alanine aminotransferase (ALT) in a mice model with the APAP-induced liver injury (AILI). Consequently, there was a dose-dependent increase in miR-122 after administration of APAP intraperitoneally. Similar observations were made for ALT activity. Additionally, the expression of miR-122 increased in a more rapid manner compared to ALT activity. However, there was a variation in the miR-122 expression. Further, we investigated the drug–drug interaction between APAP and 5-fluorouracil using miR-122 and ALT in mice. As a result, the degree of AILI was not changed by the use of 5-fluorouracil in combination with APAP in mice.