Tioguanine, also known as thioguanine or 6-thioguanine (6-TG) is a medication used to treat acute myeloid leukemia (AML), acute lymphocytic leukemia (ALL), and chronic myeloid leukemia (CML). Long-term use is not recommended. It is given by mouth.Common side effects include bone marrow suppression, liver problems and inflammation of the mouth. It is recommended that liver enzymes be checked weekly when on the medication. People with a genetic deficiency in thiopurine S-methyltransferase are at higher risk of side effects. Avoiding pregnancy when on the medication is recommended for both males and females. Tioguanine is in the antimetabolite family of medications. It is a purine analogue of guanine and works by disrupting DNA and RNA.Tioguanine was developed between 1949 and 1951. It is on the World Health Organizations List of Essential Medicines. Medical uses
Acute leukemias in both adults and children
Chronic myelogenous leukemia
Inflammatory bowel disease, especially ulcerative colitis
Psoriasis

Side effects
Leukopenia and neutropenia
Thrombocytopenia
Anemia
Anorexia
Nausea and vomiting
Hepatotoxicity: this manifests as:

Hepatic veno-occlusive disease
The major concern that has inhibited the use of thioguanine has been veno-occlusive disease (VOD) and its histological precursor nodular regenerative hyperplasia (NRH). The incidence of NRH with thioguanine was reported  as between 33 and 76%. The risk of ensuing VOD is serious and frequently irreversible so this side effect has been a major concern. However, recent evidence using an animal model for thioguanine-induced NRH/VOD has shown that, contrary to previous assumptions, NRH/VOD is dose dependent and the mechanism for this has been demonstrated.
This has been confirmed in human trials, where thioguanine has proven to be safe but efficacious for coeliac disease when used at doses below those commonly prescribed. This has led to a revival of interest in thioguanine because of its higher efficacy and faster action compared to other thiopurines and immunosuppressants such as mycophenylate. Contraindications
Pregnancy
Lactation: The safety warning against breastfeeding may have been a conservative assessment, but research evidence suggests that thiopurines do not enter breastmilk. Interactions
Cancers that do not respond to treatment with mercaptopurine do not respond to thioguanine. On the other hand, some cases of IBD that are resistant to mercaptopurine (or its pro-drug azathioprine) may be responsive to thioguanine. Pharmacogenetics
The enzyme thiopurine S-methyltransferase (TPMT) is responsible for the direct inactivation of thioguanine to its methylthioguanine base – this methylation prevents thioguanine from further conversion into active, cytotoxic thioguanine nucleotide (TGN) metabolites. Certain genetic variations within the TPMT gene can lead to decreased or absent TPMT enzyme activity, and individuals who are homozygous or heterozygous for these types of genetic variations may have increased levels of TGN metabolites and an increased risk of severe bone marrow suppression (myelosuppression) when receiving thioguanine. In many ethnicities, TPMT polymorphisms that result in decreased or absent TPMT activity occur with a frequency of approximately 5%, meaning that about 0.25% of patients are homozygous for these variants.