Trimethoprim (Oral)

As a folic acid inhibitor, trimethoprim kills bacteria that cause urinary tract infections.


Trimethoprim is used to treat initial episodes of simple urinary tract infections caused by vulnerable strains of organisms like Escherichia coli, Klebsiella pneumonia, Proteus mirabilis, Enterobacter species, and coagulase-negative Staphylococcus species (which includes S. saprophyticus). Culture and susceptibility tests may be performed to determine the sensitivity of the bacteria to trimethoprim. It does not work for viral infections, such as the common cold or the flu. Trimethoprim may interact with other drugs, such as dofetilide, which can cause a serious and life-threatening condition.

Also known by its brand names Primsol, Trimpex, and Proloprim, trimethoprim can be used in combination with other drugs to treat pneumonia, respiratory tract infections, ear infections, and traveler's diarrhea. It's most commonly used to treat urinary tract infections and the symptoms associated with it, which may include pain with urination, pelvic pain or pressure, strange-smelling urine, feeling tired or shaky, blood in the urine, cloudy or dark urine, an increased urge to urinate (and only a little comes out when you do). Some patients experience chills or fever, and this is a sign that the infection might have traveled to your kidneys.

Condition(s) Treated

  • Urinary tract infection
  • Ear infection (otitis media)
  • Pneumonia
  • Traveler's diarrhea
  • Other conditions

Type of Medicine

  • Antibiotic (synthetic antibacterial)
  • Folic acid inhibitor

Side Effects

Trimethoprim can cause some unwanted side effects, some requiring medical attention. Less common side effects include diarrhea, stomach cramps, skin rashes, nausea or vomiting, loss of appetite, or itching. Rare symptoms of trimethoprim include:

  • Discomfort or illness
  • Black, tarry stools
  • Fever (with or without chills)
  • Bluish fingernails, lips or skin
  • Headache
  • Soreness, redness, or swelling of the tongue
  • Shortness of breath or difficulty breathing
  • Unusual tiredness or weakness
  • Neck stiffness
  • Joint or muscle pain
  • Changes in color of face
  • Blood in urine or stools
  • Chills
  • Swelling
  • Pale skin
  • Unusual bruising or bleeding
  • Nausea
  • Red skin lesions (typically purple in the center)
  • Sore throat
  • Red spots on skin
  • Thick or scaly skin
  • Tenderness, peeling, redness, burning, loosening, or blistering of the skin or mucous membranes
  • Signs of electrolyte problems such as confusion, mood changes, a heartbeat that doesn't feel normal, or seizures


Before taking trimethoprim, use it as directed by your doctor. Read all of the information that comes with the medication and follow the directions carefully. Remember that every patient is different so dosing varies by patient. The amount you're prescribed is dependent on the strength of the medication. Dosage is also based on the intensity of your medical condition, as well as your response to therapy.

To treat a urinary tract infection in adults, take 100 milligrams (mg) by mouth every 12 hours for 10 days or 200 milligrams (mg) by mouth every 24 hours for 10 days.

To treat urinary tract infection in children ages 12 to 18, take 100 milligrams (mg) every 12 hours for 10 days or 200 milligrams (mg) by mouth every 24 hours for 10 days.

To make sure your infection clears up, keep taking trimethoprim for the full course of treatment even if you start feeling better after a few days. If you stop taking it before you finish the treatment, you may begin to experience symptoms again.

Trimethoprim may be taken on an empty stomach but may be taken with food if it upsets your stomach. If taken on an empty stomach, take trimethoprim one to two hours before or after your meals. Drink at least eight glasses of liquid each day while taking this medication, such as milk, fruit juice, water, coffee, tea, or soft drinks.

This medication works well when there's a continuous amount in your body. To keep a continuous amount in your body, make sure to take each dose on time and do not miss any doses. Take each dose at the same time every day to help you remember.

If you're taking more than one dose per day, space them out throughout the day and night. Taking doses at least 12 hours apart is recommended. If you need help deciding what time to take your medication, contact your doctor.

If you miss a dose, take it as soon as you remember. However, if it's near the time of the next dose, skip the missed dose and return to your normal dosing schedule. Never double up on doses.

If your condition does not improve, or it worsens after seven days of treatment, contact your doctor immediately.


Using this medication can cause adverse effects if you're taking certain other medications. Drug interactions for trimethoprim include:

  • Levomethadyl
  • Dofetilide

Other drug interactions include:

  • Acecainide
  • Lisinopril
  • Ajmaline
  • Disopyramide
  • Amiloride
  • Losartan
  • Amiodarone
  • Prajmaline
  • Amitriptyline
  • Erythromycin
  • Amoxapine
  • Lorcainide
  • Aprindine
  • Mefloquine
  • Arsenic Trioxide
  • Mercaptopurine
  • Astemizole
  • Chloral Hydrate
  • Azathioprine
  • Eprosartan
  • Azilsartan
  • Methotrexate
  • Azilsartan Medoxomil
  • Doxepin
  • Azimilide
  • Benazepril
  • Bretylium
  • Candestartan Cilexetil
  • Enalapril
  • Captopril
  • Droperidol
  • Chloroquine
  • Dolasetron
  • Enalaprilat
  • Chlorpromazine
  • Telithromycin
  • Halothane
  • Probucol
  • Cholera Vaccine, Live
  • Eltrombopag
  • Telmisartan
  • Clarithromycin
  • Enflurane
  • Desipramine
  • Hydroquinidine
  • Trandolapril
  • Dibenzepin
  • Procainamide
  • Digoxin
  • Triamterene
  • Eplerenone
  • Trifluoperazine
  • Prochlorperazine
  • Ibutilide
  • Flecainide
  • Trimipramine
  • Olmesartan Medoxomil
  • Leucovorin
  • Fluconazole
  • Valsartan
  • Propafenone
  • Fluoxetine
  • Vasopressin
  • Lidoflazine
  • Pentamidine
  • Foscarnet
  • Pyrimethamine
  • Fosinopril
  • Sotalol
  • Gemifloxacin
  • Halofantrine
  • Perindopril
  • Haloperidol
  • Spriamycin
  • Quinapril
  • Imipramine
  • Isradipine
  • Pirmenol
  • Sertindole
  • Quinidine
  • Isoflurane
  • Moexipril
  • Octreotide
  • Ramipril
  • Spironolactone
  • Nortriptyline
  • Sultopride
  • Risperidone
  • Tedisamil
  • Sematilide
  • Tedisamil
  • Zotepine
  • Zofenopril

Certain medications can cause side effects to worsen, including anisindione, amantadine, didanosine, phenytoin, fosphenytoin, tolbutamide, and repaglinide, rosiglitazone.

Ethanol can also interact with trimethoprim.

Medical problems can also cause side effects in patients using trimethoprim, some of which include kidney disease, anemia, and liver disease.


If you're allergic to trimethoprim, diuretics, sulfa drugs, or any other medications, tell your doctor or pharmacist. Be sure to let them know what nonprescription medications you're currently taking (especially phenytoin, or Dilantin). If you are taking phenytoin, your doctor may need to check your blood work more closely for as long as you're taking trimethoprim. It may be helpful to carry a list of all of your current prescriptions, vitamins, or natural products to have handy during doctor visits and trips to the pharmacy. Talk to your doctor about any vitamins you're taking beforehand.

If you have a history of kidney disease, liver disease or anemia, tell your doctor right away.

If you're pregnant or you plan on becoming pregnant, or if you're breastfeeding, let your doctor know. If you've already been prescribed trimethoprim and you think you may be pregnant, call your doctor right away. Trimethoprim can be excreted into breast milk; however, only in low levels, and not a high enough amount to cause any adverse effects. However, because of its interference with folic acid metabolism, nursing mothers should exercise caution when taking trimethoprim.

Trimethoprim may interact with certain lab tests, so tell your doctor if you plan on taking any medical tests while you're on this medication.

Do not take more than the prescribed amount of this medication because you could put yourself at a higher risk for a second urinary tract infection. Also, do not take the medication longer than advised.

If you're age 65 or older, you may have a stronger reaction to this medication, meaning the number of side effects you experience could be increased. Elderly patients who take diuretics with trimethoprim can experience blood problems.

Take caution when you're out in the sun, as trimethoprim may cause you to sunburn more easily. If you're going to be in the sun, let your doctor know so that he can help you find ways to prevent sunburn while on trimethoprim.

If your symptoms do not get better or if they worsen, call your doctor.

Never share your medication with anyone for whom it was not prescribed.

Using this medication for periods longer than intended could result in a new yeast infection or oral thrush. If you start seeing white patches in your mouth or vaginal discharge, contact your doctor.

Talk to your doctor before starting any new medication while you're taking trimethoprim, including OTC. In case of an accidental overdose, contact your local poison control center or seek emergency medical treatment immediately. Be prepared to provide information such as what, how, and when this medication was taken.

If you have any questions about trimethoprim, contact your healthcare provider and/or your pharmacist.

Trimethoprim has not been evaluated for its safety and effectiveness as a single agent in pediatric patients less than 12 years of age; in pediatric patients less than 2 months of age; or for the treatment of acute Otis media in patients less than 6 months of age.

Trimethoprim does not treat viral infections; it only treats bacterial infections. Do not take trimethoprim to treat a viral infection.

If you develop anemia while taking trimethoprim, your doctor may instruct you to take folic acid every day to help overcome the anemia.

If you have an intolerance to some sugars, talk to your doctor before you take trimethoprim because it contains a sugar called lactose.

Failing to complete the full course of medicinal therapy can result in a bacterial resistance to the medication. You also risk decreasing the effectiveness of the drug the second time you attempt to get rid of the infection.

Diarrhea is common in patients taking trimethoprim. In rare cases, severe diarrhea, also known as Clostridium difficile (C-diff)--associated diarrhea (CDAD), can occur. In some cases, it leads to colitis, a life-threatening bowel condition. CDAD can happen while you're on trimethoprim, or months after you've stopped the medication. Seek advice from your doctor immediately if you experience bloody stools, very watery or loose stools, or stomach cramps or pain. Do not try to self-diagnose and treat the loose stools without talking to your doctor.

Some patients experience aseptic meningitis while taking trimethoprim and it usually presents itself in the form of the following symptoms: stiff neck, headache, fever, tiredness, sensitivity to bright light and feeling ill.

Trimethoprim increases your risk of certain blood problems and certain infections, bleeding of the gums, and slow healing. Therefore, you should take caution when using toothbrushes, toothpicks, and dental floss. Delay any dental work until your blood count reaches a normal level. Talk to your doctor if you have any questions about proper oral hygiene while taking this medication.

Avoid the use of tanning beds and sunlamps and be sure to use sunscreen (preferably an SPF 15 or higher) because some patients experience skin rashes or severe sunburn. It's also best to stay out of direct sunlight between 10am and 3pm, if possible. If you do experience severe reactions from the sun, call your doctor.


Trimethoprim should be kept in a tightly closed container out of reach and sight of children. It should be stored at room temperature (below 25 degrees Fahrenheit) and kept away from extreme moisture and excess heat--which means the bathroom is not an ideal place to store it. Make sure you keep it in a place where pets or other family members cannot easily access them.

Throw out any expired or unused medication; however, avoid flushing it down the toilet or throwing it in the trash. If you're unsure about how to dispose of your unused or expired medication, talk to your doctor or pharmacist. Your community may have a program that accepts old or unused medication.

Do not take it past the expiration date. The expiration date is usually the last date of the month. Contact your doctor or pharmacist about refills, if needed.


Trimethoprim treats conditions and symptoms associated with urinary tract infections, ear infections, pneumonia, and traveler's diarrhea. It treats symptoms such as pain with urination, pelvic pain or pressure, strange-smelling urine, feeling tired or shaky, blood in the urine, cloudy or dark urine, and an increased urge to urinate (with only a little passing when you do). When combined with other medications, this medication can cause an increase in side effects, such as fever (with or without chills); black, tarry stools; bluish fingernails, lips or skin; headache; soreness, redness, or swelling of the tongue; shortness of breath or difficulty breathing; unusual tiredness or weakness; neck stiffness; joint or muscle pain; changes in color of face; and blood in the urine or stools.

It's important that patients tell their doctor exactly what prescription and nonprescription medications they are taking to avoid any major drug interactions. You also need to talk to your doctor about what conditions you might have to prevent any increase in side effects that may occur. Patients taking trimethoprim can experience an increased sensitivity to sunlight, meaning it should be avoided while taking this medication (if possible), or severely limited. Elderly patients (over age 65) might have an increase in blood problems while using this medication. It's recommended that you take the full course of treatment and if the problem recurs, contact your doctor. Together, you can determine what the next steps should be: either another round of treatment or alternative treatment options.

Last Reviewed:
December 24, 2017
Last Updated:
April 05, 2018
Content Source: