Diminished synthesis of purines and thymidines which leads to the inability of the cells to divide.

Patients with megaloblastic anemia have an increased demand for folic acid

Patients with megaloblastic anemia have poor absorption of folic acid due to a pathology of the msall intestine, alcoholism, or treatment with drugs that are dihydrofolate reductase inhibitors (like methotrexate)

Erythropoietic cells of the bone marrow and the mucosal cells of the intestine (rapidly dividing cells) have a high demande for N5,N10-methylene and N10-formyl forms of THF. N5-methylene form cannot be converted to other forms without B12, so N5 form accumulates whereas the levels of other forms decrease leading to megaloblastic anemia.