Methotrexate is a medication that acts as a nucleic acid synthesis inhibitor, specifically targeting the synthesis of purines and pyrimidines, which are essential building blocks for DNA and RNA.MECHANISM1.Folate Metabolism: Folate (vitamin B9) is essential for the synthesis of nucleic acids (DNA and RNA) because it provides one-carbon units for the synthesis of purines and pyrimidines.2.DHFR Inhibition:Methotrexate enters the cell and inhibits the enzyme dihydrofolate reductase (DHFR).• DHFR is responsible for converting dihydrofolate (DHF) into tetrahydrofolate (THF), an active form of folate.•By inhibiting DHFR, methotrexate reduces the production of THF.3.Reduced Nucleotide Synthesis: With reduced levels of THF, the synthesis of purines (adenine and guanine) and pyrimidines (thymine, cytosine, and uracil) is impaired.• Purines and pyrimidines are essential components of DNA and RNA.•The reduced availability of these nucleotides slows down DNA and RNA synthesis in rapidly dividing cells.4.Inhibition of Cell Division:Methotrexate's inhibition of nucleotide synthesis affects rapidly dividing cells, such as cancer cells and cells involved in inflammatory processes.•Without sufficient nucleotides, cells cannot replicate their DNA properly, leading to inhibition of cell division.•This mechanism is particularly effective in targeting cancer cells, which rely heavily on rapid DNA synthesis and cell division.•Methotrexate's is used in the treatment of cancer, autoimmune diseases (such as rheumatoid arthritis), and other conditions where suppression of cell division and inflammation is desired.