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Drug Info

What are the side effects of Bactrim for older adults?

If you’re older than 65, there are some important risks of this Rx
Racheal Ede
By | Jan. 18, 2024 Medically reviewed by Karen Berger, Pharm.D.
A doctor and an older adult | Bactrim side effects in elderly
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If you’ve ever had a bacterial infection, it’s possible your healthcare provider prescribed a medication called Bactrim to treat it. Bactrim (or Bactrim DS for the double-strength version)—which is also sold as Sulfatrim or Septra—is the brand name for a combination medication containing two antibiotics: sulfamethoxazole and trimethoprim. It is used to treat various bacterial infections, such as urinary tract infections, bronchitis, traveler’s diarrhea, and pneumonia. 

Although Bactrim is considered safe, it can trigger serious adverse effects in older adults, such as hyperkalemia, blood disorders, hypoglycemia, and neurological confusion. Here, we outline all you need to know about the possible side effects of Bactrim in those 65 and older along with precautions you can take for safe and effective use. 

Common Bactrim side effects

Bactrim, sometimes called co-trimoxazole, is well-tolerated by most people, but like most drugs, it can have unwanted side effects. Common side effects of Bactrim for people of any age include:

  • Loss of appetite
  • Tiredness
  • Nausea and vomiting
  • Painful or swollen tongue
  • Dizziness
  • Ringing in your ears
  • Fatigue
  • Trouble sleeping
  • Skin rashes
  • Photosensitivity—heightened skin reactions on exposure to the sun

Bactrim side effects in elderly people

People 65 years of age and older are more susceptible to the side effects of sulfamethoxazole/trimethoprim. Because people of that age are more likely to have folate deficiency and age-related kidney or liver problems, they have an increased risk of side effects—and more serious side effects. The following are five serious side effects that may occur with Bactrim use in the elderly: 

1. Hyperkalemia

Hyperkalemia is increased potassium levels in the blood. Older patients are at risk of hyperkalemia when taking Bactrim if they are on drugs predisposing them to hyperkalemia, if they have problems metabolizing potassium, or if they have kidney dysfunction. 

A large 18-year study found that senior patients taking Bactrim and spironolactone concurrently were at increased risk of being admitted to the hospital for very high potassium levels. “Trimethoprim, one of the components of Bactrim, inhibits potassium excretion in the kidneys, leading to an accumulation of potassium in the body,” says Whitney Prude, Pharm.D., a board-certified pharmacotherapy specialist based in Rochester, Minnesota. “This can be particularly concerning in older adults who may already have compromised kidney function and are taking other medications that also affect potassium levels like angiotensin-converting enzyme (ACE) inhibitors or angiotensin receptor blockers (ARBs).” 

High potassium levels can cause irregular heart rhythms and sudden death if not treated immediately. Seek urgent treatment or call 911 if you have symptoms including shortness of breath, confusion, weakness, muscle twitching, irregular heartbeat, numbness or tingling in the hands, feet, or lips—as these may be symptoms of hyperkalemia, which is sometimes accompanied by hyponatremia (low sodium). 

2. Blood disorders

Older adults are often deficient in folate, and people with folate deficiency are at risk of blood disorders if they take Bactrim—both of which increase their risk of blood disorders such as:

  • Hemolytic anemia occurs when red blood cells are destroyed faster than they are made. People with an enzyme deficiency known as glucose-6-phosphate dehydrogenase (an enzyme that protects the red blood cells) are at risk of hemolytic anemia.
  • Megaloblastic anemia is a type of anemia characterized by abnormally large red blood cells.
  • Thrombocytopenia is a condition that causes reduced platelet cells, which may lead to bleeding problems. People taking thiazide diuretics, such as hydrochlorothiazide, are at an increased risk.
  • Agranulocytosis is a life-threatening condition in which the bone marrow is unable to produce enough neutrophils (the white blood cells that fight infection in the body).

3. Hypoglycemia

Hypoglycemia occurs when a person has low blood sugar levels. Older adults are at higher risk of hypoglycemia when taking Bactrim if they have kidney dysfunction, liver disease, or if they are malnourished.

4. Digoxin toxicity

Digoxin is a medication used to treat certain heart conditions such as failure and atrial fibrillation. Older age, kidney disease, and certain types of medications can increase digoxin blood levels. “Studies have shown that the coadministration of digoxin and trimethoprim resulted in increases of 30% to 50% of the drug in elderly patients,” Dr. Prude says. This is because Bactrim blocks the kidneys from clearing digoxin, which causes increased levels in the body. Small increases in blood levels of digoxin can cause life-threatening toxicity.

According to Dr. Prude, symptoms of high levels of digoxin include: 

  • Appetite loss, nausea, vomiting, or diarrhea
  • Headache, confusion, anxiety, or hallucinations
  • Restlessness, weakness, or depression
  • Changes in vision such as blurred vision or seeing halos around bright objects
  • A fast, slow, or irregular heartbeat, or palpitations (fast, forceful heartbeats in an irregular rhythm)

If you notice symptoms associated with digoxin toxicity, get medical care right away.

5. Neurological confusion

Bactrim crosses the blood-brain barrier and may cause adverse neurological reactions in older people. However, this is a rare side effect that would likely resolve after the drug is discontinued. For example, there is a case report of delirium (serious confusion or disorientation) and gait disturbances (abnormal walking, poor balance, and constant falls) in an 82-year-old patient treated with high doses of Bactrim. 

Risk factor analysis for Bactrim side effects in seniors

“Bactrim is not contraindicated in older people based on age alone,” says Patricia Weiser, Pharm.D., a contributor to SingleCare’s The Checkup. “However, older people (ages 65 years and up) have a higher risk of developing potentially severe side effects from Bactrim, such as high potassium levels. Healthcare providers may choose to prescribe Bactrim for an older adult if they feel the benefits outweigh the risks.”

Before prescribing Bactrim, your healthcare provider will take your medical history and other medications you take into account. Certain drugs interact with Bactrim; in some cases, the combination cannot be prescribed together, and in others, close monitoring would be required. These meds include:

There is also the risk of drug-condition interaction if you have medical conditions such as porphyria, severe asthma, diabetes, thyroid problems, group A β-hemolytic streptococcal infections, or glucose-6-phosphate dehydrogenase deficiency. Other reasons they would avoid prescribing Bactrim include if you have a folate deficiency, a known hypersensitivity to trimethoprim or sulfonamides (any sulfa drug), a history of drug-induced low platelet levels with the use of trimethoprim and/or sulfonamides, or a certain degree of liver or kidney problems.

Precautions for Bactrim use in older adults

If you are prescribed Bactrim, it’s important to know the precautions to take as an older adult. The following are other tips for safe and effective use of Bactrim: 

  • Inform your healthcare provider of all prescription drugs, over-the-counter medicines, vitamins, and supplements you are taking. It’s essential that your healthcare provider know everything that you are taking before they prescribe Bactrim, as it can have interactions. 
  • Follow the instructions on your prescription label. Start by reading the package insert, and ask your pharmacist questions if there’s anything you don’t understand. 
  • Stay hydrated. Be sure to drink enough water when taking Bactrim to prevent kidney stone formation or crystals in urine. 
  • Watch out for diarrhea. Antibiotic-associated diarrhea, which can occur with any antibiotic, may develop up to several months after discontinuing the medication. Reach out to your healthcare provider right away if you start having symptoms such as watery and bloody stools.
  • Watch for symptoms of hyperkalemia if you take certain medications. Patients with heart failure who take spironolactone (or other meds that raise blood potassium, including ACE inhibitors or ARBs) should be monitored regularly to ensure that potassium levels do not rise to dangerously high levels, as this is life-threatening. 
  • Complete the full course of antibiotics. Even if you start to feel better, it is important to finish taking the medication as prescribed. By not completing your antibiotic dose, the effectiveness of the treatment will be reduced, and the bacteria may become resistant to Bactrim or even other antibiotics. 
  • Follow up with regular lab testing. Depending on your risk factors and the length of your Bactrim treatment, your doctor may order regular laboratory blood tests, including complete blood cell counts and clinical chemistry tests evaluating liver and kidney function.

Alternatives for Bactrim

If you cannot take Bactrim because of an allergy or one of the reasons listed above, the alternative antibiotic prescribed will vary depending on the condition. “There is a very long list of drugs that could be used in place of Bactrim, but it would depend on the type of bacteria, the location of the infection, and what the guideline-recommended agents would be for treating those specific types of infections,” Dr. Prude says.

Three common alternatives include:

  • Ciprofloxacin, an alternative for treating UTIs in older patients. According to one study, ciprofloxacin, a quinolone antibiotic, has a higher cure rate of 96%, compared to 87% for Bactrim. Ciprofloxacin also showed fewer side effects. 
  • Levofloxacin, another option for infections in older adults. This quinolone antibiotic may be preferred over ciprofloxacin because it has a lower probability of drug reactions. 
  • Cefuroxime, a cephalosporin antibiotic that can be used to treat many types of bacterial infections. 

The bottom line: Is Bactrim safe for elderly people?

Bactrim use in older adults has been associated with rare but serious adverse reactions, including neurological confusion, hyperkalemia, blood disorders, and hypoglycemia. If your provider suggests Bactrim as a potential treatment after age 65, it’s important to weigh the benefits against the risks—especially if you have age-related kidney, liver, or heart diseases. You may need dose adjustment or an alternative to Bactrim. Always seek medical advice from a healthcare professional for any unusual symptoms when using Bactrim.