Edward Eric Schmidt has written:

'Repression of dihydrofolate reductase synthesis during myogenesis' -- subject(s): Genetic regulation, Myogenesis

Competitive inhibitor is a substance that competes directly with a normal substrate for an enzymatic-binding site of an enzyme. Such an inhibitor usually resembles the substrate to the extent that it specifically binds to the active site of the enzyme but differs from it so as to be unreactive and therefore there will be no catalytic reaction. Some examples are: methotrexate that is competitive inhibitor of dihydrofolate reductase, and malonate which structurally resembles succinate (that is converted to fumarate by succinate dehydrogenase during the citric acid cycle) but cannot be dehydrogenated.

I'm a little confused by "mercury drug", since methotrexate does not actually contain mercury. The IUPAC name is (2S)-2-[(4-{[(2,4-diaminopteridin-6-yl)methyl](methyl)amino}benzoyl)amino]pentanedioic acid, which probably doesn't mean much to you unless you're a real organic chemistry geek, but if you're asking what it's for... it's mainly an antineoplastic, though it's also used in lower doses for autoimmune diseases and as an abortifacient. It's structurally very similar to dihydrofolic acid, so it acts as an inhibitor for dihydrofolate reduction and effectively shuts down a crucial metabolic pathway for the synthesis of DNA, RNA, and purines generally.

Source: rx2040. com

Methotrexate is an antimetabolite used to treat certain types of cancer.

Methotrexate is an antimetabolite used to treat certain types of cancer. It inhibits dihydrofolate reductase, an important enzyme responsible for the synthesis of nucleotides for DNA. This medication inhibits synthesis of DNA, DNA reparation and cell mitosis. Cells of malignant tumor, bone marrow, embryonic cells, epithelial cells of the intestinal mucosa, bladder, oral cavity are the most sensitive to it.

Methotrexate is used in treatment of breast cancer, squamous cell carcinoma of the head and neck, lung cancer, trophoblastic tumor, cervical cancer, ovarian cancer, bladder cancer, colorectal cancer, esophagus cancer, stomach cancer, pancreatic cancer, testicular cancer, acute lymphoblastic leukemia and many others.