DNA methylation is an important component of epigenetics that is involved in the occurrence and development of a variety of diseases. The present study aimed to clarify the relationship between cytochrome P450 (CYP)1A1 and CYP1B1 promoter CpG island methylation and isoniazid‑induced liver injury in rats, and to explore the possible mechanism, rats were given an intragastric dose of isoniazid (55 mg·kg‑1·d‑1). High performance liquid chromatography was used to analyze the DNA methylation level of the whole genome in liver tissue. Methylation‑specific polymerase chain reaction (PCR) was used to detect the methylation level of CpG islands in the promoter region of CYP1A1 and CYP1B1. Reverse transcription‑quantitative PCR was used to determine the mRNA expression levels of CYP1A1, CYP1B1, toll‑like receptor 4 (TLR4), extracellular signal‑regulated kinase (ERK) 2, peroxisome proliferator‑activated receptor (PPAR) ‑γ, interleukin (IL)‑6 and tumor necrosis factor (TNF)‑α. The expression levels of CYP1A1 and CYP1B1 proteins were measured by ELISA, and malondialdehyde (MDA) content and superoxide dismutase (SOD) activity were analyzed by colorimetric method. Liver tissue pathology, an indicator of liver function, indicated rat liver injury at 10 days following isoniazid treatment. Whole‑genome methylation levels were gradually reduced, and methylation at day 7 post‑treatment was significantly lower than the control group. CYP1A1 and CYP1B1 promoter CpG island methylation level was significantly increased at 3 days post‑treatment. CYP1A1 and CYP1B1 mRNA expression levels were significantly reduced from day 7 and 10, respectively. These results suggested that CpG island hypermethylation of the CYP1A1 and CYP1B1 promoters regulate the low expression of genes involved in the occurrence of isoniazid‑induced liver injury. With the alterations of CYP1A1 and CYP1B1 expression, the mRNA expression levels of TLR4, ERK, MDA, IL‑6 and TNF‑α were upregulated, and the expression of SOD and PPAR‑γ were downregulated. These data demonstrated that alterations in methylation patterns may involve changes in the TLR4‑ERK signaling pathway and PPAR‑γ, which may alter the expression of MDA, SOD, IL‑6 and TNF‑α, leading to liver injury.