Thioguanine belongs to the group of cancer-fighting medications known as antineoplastics, and specifically to the group of antineoplastics known as antimetabolites. Thioguanine fights cancer by preventing the growth of cancer cells, which eventually kills them. Thioguanine is usually used to treat leukemia.

This medication may be available under multiple brand names and/or in several different forms. Any specific brand name of this medication may not be available in all of the forms or approved for all of the conditions discussed here. As well, some forms of this medication may not be used for all of the conditions discussed here.

Your doctor may have suggested this medication for conditions other than those listed in these drug information articles. If you have not discussed this with your doctor or are not sure why you are taking this medication, speak to your doctor. Do not stop taking this medication without consulting your doctor.

Do not give this medication to anyone else, even if they have the same symptoms as you do. It can be harmful for people to take this medication if their doctor has not prescribed it.

The recommended dose of thioguanine varies according to the specific condition being treated, the response to therapy, the other medications used, and the stage of disease. The dose of thioguanine to be given also depends on body weight. A typical starting dose for adults is 2 mg per kg of body weight per day taken once daily.

Many things can affect the dose of medication that a person needs, such as body weight, other medical conditions, and other medications. If your doctor has recommended a dose different from the ones listed here, do not change the way that you are taking the medication without consulting your doctor.

It is important to take this medication exactly as prescribed by your doctor. It may cause nausea and vomiting, but you should continue to take it, even if you start to feel ill. Do not stop taking thioguanine without talking to your doctor. If you vomit shortly after taking it, contact your doctor for instruction on whether to take more medication or not. If you miss a dose, skip the missed dose and go back to your regular dosing schedule. Do not take a double dose to make up for a missed one. If you miss more than 2 doses of thioguanine, contact your doctor.

Your doctor may want you to drink extra fluids while taking this medication in order to help you pass more fluid and protect your kidneys. Keep track of any side effects and report them to your doctor as suggested in the section "What side effects are possible with this medication?"

Store this medication at room temperature, protect it from excessive heat, direct light, and moisture, and keep it out of the reach of children.

Each pale, greenish-yellow, biconvex tablet, plain on one side and scored on the other side with "Wellcome" on the upper half and "U3B" on the lower half, contains 40 mg of thioguanine. Nonmedicinal ingredients: gum acacia, lactose, magnesium stearate, potato starch, and stearic acid.

Do not take this medication if you:

• are allergic to thioguanine or any ingredients of this medication
• have been resistant to the effects of thioguanine or mercaptopurine in the past

Many medications can cause side effects. A side effect is an unwanted response to a medication when it is taken in normal doses. Side effects can be mild or severe, temporary or permanent.

The side effects listed below are not experienced by everyone who takes this medication. If you are concerned about side effects, discuss the risks and benefits of this medication with your doctor.

The following side effects have been reported by at least 1% of people taking this medication. Many of these side effects can be managed, and some may go away on their own over time.

Contact your doctor if you experience these side effects and they are severe or bothersome. Your pharmacist may be able to advise you on managing side effects.

• diarrhea
• loss of appetite
• nausea and vomiting
• skin rash and itching
• sun sensitivity

Although most of the side effects listed below don't happen very often, they could lead to serious problems if you do not seek medical attention.

Check with your doctor as soon as possible if any of the following side effects occur:

• joint pain or swelling
• sores in the mouth or on the lips
• signs of anemia (low red blood cells; e.g., dizziness, pale skin, unusual tiredness or weakness, shortness of breath)
• signs of bleeding (e.g., bloody nose, blood in urine, coughing blood, bleeding gums, cuts that don't stop bleeding)
• signs of infection (fever, severe chills, sore throat, mouth ulcers)
• signs of abnormal liver function including skin or whites of the eyes turning yellow, feeling tired, dark or brown coloured urine, nausea or vomiting or not wanting to eat)
• swelling of the feet or lower legs
• unsteadiness when walking

Contact your doctor immediately if any of the following side effects occur:

• signs of bleeding in the stomach (e.g., bloody, black, or tarry stools, spitting up of blood, vomiting blood or material that looks like coffee grounds)
• signs of a serious allergic reaction (e.g., abdominal cramps, difficulty breathing, nausea and vomiting, or swelling of the face and throat)
• signs of serious stomach problems (e.g., severe diarrhea, bloody stools, bloating or swelling, or severe abdominal pain)

Some people may experience side effects other than those listed. Check with your doctor if you notice any symptom that worries you while you are taking this medication.

Before you begin using a medication, be sure to inform your doctor of any medical conditions or allergies you may have, any medications you are taking, whether you are pregnant or breast-feeding, and any other significant facts about your health. These factors may affect how you should use this medication.

Anemia: Thioguanine may cause low levels of red blood cells. If you experience symptoms of reduced red blood cell count (anemia) such as shortness of breath, feeling unusually tired, or pale skin, contact your doctor as soon as possible

Blood clotting: This medication can reduce the number of platelet cells in the blood. Platelets help the blood to clot, and a shortage could make you bleed more easily. Tell your doctor of any signs that your blood is not clotting as quickly as usual. Such symptoms may include black and tarry stools, blood in the urine, easy bruising, or cuts that won't stop bleeding.

Infection: As well as killing cancer cells, this medication can reduce the number of cells that fight infection in the body (white blood cells). Avoid contact with people with contagious infections and tell your doctor if you begin to notice signs of an infection, such as fever or chills.

Liver function: Thioguanine can reduce liver function and can cause liver damage. If you have liver problems, discuss with your doctor how this medication may affect your medical condition, how your medical condition may affect the dosing and effectiveness of this medication, and whether any special monitoring is needed. Your doctor will order blood tests for your liver function regularly while you are taking this medication.

If you experience symptoms of liver problems such as fatigue, feeling unwell, loss of appetite, nausea, yellowing of the skin or whites of the eyes, dark urine, pale stools, abdominal pain or swelling, and itchy skin, contact your doctor immediately.

Pregnancy: There is a possibility of birth defect if either the man or woman is using thioguanine at the time of conception. As well, it may harm the baby if used during pregnancy. Effective birth control should be practiced while using this medication.

This medication should not be used during pregnancy unless the benefits outweigh the risks. If you become pregnant while taking this medication, contact your doctor immediately.

Breast-feeding: It is not known whether thioguanine passes into breast milk. Because of the risks associated with this medication, a decision should be made to stop breast-feeding or to stop the medication, taking into account the importance of the medication to the mother.

There may be an interaction between thioguanine and any of the following:

• aminosalicylate drugs (e.g., sulfasalazine, mesalazine, olsalazine)
• bacillus Calmette-Guérin (BCG)
• clozapine
• deferiprone
• denosumab
• echinacea
• medications that suppress the immune system:
• corticosteroids (e.g., dexamethasone, hydrocortisone, prednisone)
• medications used to treat conditions such as cancer, inflammatory bowel disease, multiple sclerosis, psoriasis, rheumatoid arthritis, or medications used after a transplant
• other cancer medications
• vaccines

If you are taking any of these medications, speak with your doctor or pharmacist. Depending on your specific circumstances, your doctor may want you to:

• stop taking one of the medications,
• change one of the medications to another,
• change how you are taking one or both of the medications, or
• leave everything as is.

An interaction between two medications does not always mean that you must stop taking one of them. Speak to your doctor about how any drug interactions are being managed or should be managed.

Medications other than those listed above may interact with this medication. Tell your doctor or prescriber about all prescription, over-the-counter (non-prescription), and herbal medications you are taking. Also tell them about any supplements you take. Since caffeine, alcohol, the nicotine from cigarettes, or street drugs can affect the action of many medications, you should let your prescriber know if you use them.