In the U.S.—

In Canada—

• Antibacterial, systemic

Trimethoprim (trye-METH-oh-prim) is used to treat infections of the urinary tract. It may also be used for other problems as determined by your doctor. It will not work for colds, flu, or other virus infections.

Trimethoprim is available only with your doctor's prescription, in the following dosage form:

Oral
• Tablets (U.S. and Canada)

In deciding to use a medicine, the risks of taking the medicine must be weighed against the good it will do. This is a decision you and your doctor will make. For trimethoprim, the following should be considered:

Allergies—Tell your doctor if you have ever had any unusual or allergic reaction to trimethoprim. Also tell your health care professional if you are allergic to any other substances, such as foods, preservatives, or dyes.

Pregnancy—Studies have not been done in humans. Studies in rats have shown that trimethoprim causes birth defects. Studies in rabbits have shown that trimethoprim causes a decrease in the number of successful pregnancies. However, in the few reports where trimethoprim was taken by pregnant women, trimethoprim has not been reported to cause birth defects or other problems in humans.

Breast-feeding—Trimethoprim passes into the breast milk. However, this medicine has not been reported to cause serious problems in nursing babies.

Children—This medicine has been used in a limited number of children 2 months of age or older, and tested in children 12 years of age or older. In effective doses, the medicine has not been shown to cause different side effects or problems in children than it does in adults.

Older adults—Elderly people may be more sensitive to the effects of trimethoprim. Blood problems may be more likely to occur in elderly patients who are taking diuretics (water pills) along with this medicine.

Other medicines—Although certain medicines should not be used together at all, in other cases two different medicines may be used together even if an interaction might occur. In these cases, your doctor may want to change the dose, or other precautions may be necessary. When you are taking trimethoprim, it is especially important that your health care professional know if you are taking any of the following:

• Anticonvulsants (seizure medicine) or
• Methotrexate (e.g., Mexate) or
• Pyrimethamine (e.g., Daraprim) or
• Triamterene (e.g., Dyrenium)—Use of these medicines with trimethoprim may increase the chance of side effects affecting the blood

• Anemia—Patients with anemia may have an increased chance of side effects affecting the blood

• Kidney disease—Patients with kidney disease may have an increased chance of side effects

• Liver disease—Patients with liver disease may have an increased chance of side effects

Do not give this medicine to infants or children under 12 years of age unless otherwise directed by your doctor.

Trimethoprim may be taken on an empty stomach or, if it upsets your stomach, it may be taken with food.

To help clear up your infection completely, keep taking this medicine for the full time of treatment even if you begin to feel better after a few days. If you stop taking this medicine too soon, your symptoms may return.

This medicine works best when there is a constant amount in the body. To help keep the amount constant, do not miss any doses. Also, it is best to take the doses at evenly spaced times day and night. For example, if you are to take 2 doses a day, the doses should be spaced about 12 hours apart. If this interferes with your sleep or other daily activities, or if you need help in planning the best times to take your medicine, check with your health care professional.

Dosing—

The dose of trimethoprim will be different for different patients. Follow your doctor's orders or the directions on the label. The following information includes only the average doses of trimethoprim. Your dose may be different if you have kidney disease. If your dose is different, do not change it unless your doctor tells you to do so.

The number of tablets that you take depends on the strength of the medicine. Also, the number of doses you take each day, the time allowed between doses, and the length of time you take the medicine depend on the medical problem for which you are taking trimethoprim.

• For the treatment of urinary tract infections :
  • Adults and children 12 years of age and older: 100 milligrams every twelve hours for ten days, or 200 milligrams once a day for ten days.
  • Children up to 12 years of age: Dose must be determined by the doctor.

Missed dose—

If you do miss a dose of this medicine, take it as soon as possible. This will help to keep a constant amount of medicine in the body. However, if it is almost time for your next dose, skip the missed dose and go back to your regular dosing schedule. Do not double doses.

Storage—

To store this medicine:

• Keep out of the reach of children.
• Store away from heat and direct light.
• Do not store in the bathroom, near the kitchen sink, or in other damp places. Heat or moisture may cause the medicine to break down.
• Do not keep outdated medicine or medicine no longer needed. Be sure that any discarded medicine is out of the reach of children.

It is important that your doctor check your progress at regular visits if you will be taking this medicine for a long time. This will allow your doctor to check for any unwanted effects that may be caused by this medicine.

If your symptoms do not improve within a few days, or if they become worse, check with your doctor.

If this medicine causes anemia, your doctor may want you to take folic acid (a vitamin) every day to help clear up the anemia. If so, it is important to take folic acid every day along with this medicine; do not miss any doses.

Trimethoprim may cause blood problems. These problems may result in a greater chance of certain infections, slow healing, and bleeding of the gums. Therefore, you should be careful when using regular toothbrushes, dental floss, and toothpicks. Dental work should be delayed until your blood counts have returned to normal. Check with your medical doctor or dentist if you have any questions about proper oral hygiene (mouth care) during treatment.

Some people who take trimethoprim may become more sensitive to sunlight than they are normally. Exposure to sunlight, even for brief periods of time, may cause severe sunburn or skin rash, redness, itching, or discoloration. When you begin taking this medicine:

• Stay out of direct sunlight, especially between the hours of 10:00 a.m. and 3:00 p.m., if possible.
• Apply a sun block product that has a skin protection factor (SPF) of at least 15. Some patients may require a product with a higher SPF number, especially if they have a fair complexion. If you have any questions about this, check with your health care professional.
• Do not use a sunlamp or tanning bed or booth.

If you have a severe reaction from the sun, check with your doctor.

Along with its needed effects, a medicine may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.

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

• Less common
  • Skin rash or itching 
  
  

• Rare
  • Black, tarry stools;  blood in urine or stools;  bluish fingernails, lips, or skin;  changes in facial skin color;  chills ;  difficult breathing or shortness of breath;  fever with or without chills;  general feeling of discomfort or illness;  headache;  joint or muscle pain;  nausea;  neck stiffness;  pale skin;  pinpoint red spots on skin;  redness, blistering, burning, tenderness, peeling, or loosening of skin or mucous membranes ;  redness, swelling, or soreness of tongue;  red skin lesions, often with a purple center;  sore throat;  swelling;  thickened or scaly skin;  unusual bleeding or bruising ;  unusual tiredness or weakness 
  
  

Other side effects may occur that usually do not need medical attention. These side effects may go away during treatment as your body adjusts to the medicine. However, check with your doctor if any of the following side effects continue or are bothersome:

• Less common
  • Diarrhea;  loss of appetite;  nausea or vomiting;  stomach cramps or pain 
  
  

Other side effects not listed above also may occur in some patients. If you notice any other effects, check with your doctor.

Once a medicine has been approved for marketing for a certain use, experience may show that it is also useful for other medical problems. Although these uses are not included in product labeling, trimethoprim is used in certain patients for the following medical conditions:

• Prevention of urinary tract infections
• Treatment of Pneumocystis carinii pneumonia (PCP)

For patients taking this medicine for prevention of urinary tract infections:

• Your doctor may have prescribed this medicine to prevent infections of the urinary tract. It is usually given once a day and may be given for a long time for this purpose. If you have any questions about this, check with your doctor.

Other than the above information, there is no additional information relating to proper use, precautions, or side effects for these uses.