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Antibiotics first came into general use with soldiers during World War II. Since then, antibiotics have saved millions of lives. But like all drugs, antibiotics have side effects. One of the most common is antibiotic-associated diarrhea (AAD) — a potentially serious condition that affects up to one in five people receiving antibiotic therapy.

Antibiotic-associated diarrhea occurs when antibiotics disturb the natural balance of "good" and "bad" bacteria in your intestinal tract, causing harmful bacteria to proliferate far beyond their normal numbers. The result is often frequent, watery bowel movements.

Most often, antibiotic-associated diarrhea is fairly mild and clears up shortly after you stop taking the antibiotic. But sometimes you may develop colitis, an inflammation of your colon, or a more serious form of colitis called pseudomembranous colitis. Both can cause abdominal pain, fever and bloody diarrhea. In cases of pseudomembranous colitis, these symptoms may become life-threatening.

Effective treatments exist for mild antibiotic-associated diarrhea, as well as for colon inflammation. In addition, taking concentrated supplements of beneficial bacteria (probiotics) or eating yogurt may relieve symptoms or help prevent antibiotic-associated diarrhea in the first place.

Antibiotic-associated diarrhea can cause signs and symptoms that range from mild to severe. Most often, you'll have only a slight change in the bacteria in your digestive tract, which can cause loose stools or more bowel movements a day than are normal for you.

If you develop these symptoms, they're likely to begin about five to 10 days after you start antibiotic therapy and to end within two weeks after you stop taking antibiotics. Sometimes, however, diarrhea and other symptoms may not appear for days or even weeks after you've finished antibiotic treatment.

When the overgrowth of harmful bacteria is severe, you may have signs and symptoms of colitis or pseudomembranous colitis, such as:

• Frequent, watery diarrhea
• Abdominal pain and cramping
• Fever, often higher than 101 F
• Pus in your stool
• Bloody stools
• Nausea
• Dehydration

Most people experience an improvement within two weeks of beginning treatment for colitis or pseudomembranous colitis. However, sometimes symptoms recur — usually within a month of the initial treatment — which means you may need to be treated a second or even a third time.

Your digestive tract is a complex ecosystem that's home to millions of microorganisms (intestinal flora), including more than 500 species of bacteria. Many of these bacteria are beneficial, performing essential functions such as synthesizing certain vitamins, stimulating your immune system and helping protect you from harmful viruses and bacteria.

But some of the bacteria that normally inhabit your intestinal tract are potentially dangerous. They're usually kept in check by beneficial bacteria unless the delicate balance between the two is disturbed by illness, medications or other factors.

Antibiotics can be especially disruptive to intestinal flora, the bacteria that normally live in the colon, because they destroy beneficial bacteria along with harmful ones. Without enough "good" microorganisms, "bad" bacteria that are resistant to the antibiotic you received grow out of control, producing toxins that can damage the bowel wall and trigger inflammation.

The usual suspect
The bacterium responsible for almost all cases of pseudomembranous colitis and many instances of severe antibiotic-associated diarrhea is Clostridium difficile. Most peopleacquire the infection during a stay in a hospital or nursing home after they've received antibiotics.

It's likely that many hospitalized people are exposed to C. difficile, but the bacterium causes problems only in people treated with antibiotics. Then it grows out of control, leading to severe diarrhea and potentially life-threatening complications.

Which antibiotics cause AAD?
Virtually any antibiotic can cause antibiotic-associated diarrhea, colitis or pseudomembranous colitis, but the most common culprits are ampicillin and clindamycin.

Other antibiotics that have been especially implicated in pseudomembranous colitis include third-generation cephalosporins such as cefpodoxime (Vantin). Sometimes erythromycins (Erythrocin), fluoroquinolones (Cipro, Floxin) and tetracyclines also can cause antibiotic-associated diarrhea. Problems can occur whether you take the antibiotics by mouth or receive them by injection.

Other effects of antibiotics
In addition to disrupting the balance of microorganisms in your digestive tract, antibiotics can also affect the following:

• Rate of digestion. Antibiotics such as erythromycin can cause food to leave your stomach too quickly, causing nausea and vomiting. Other antibiotics may increase intestinal contractions, speeding up the rate at which food moves through your small intestine and contributing to diarrhea.
• Breakdown of food. Antibiotics may affect the way your body metabolizes fatty acids.

Anyone who undergoes antibiotic therapy is at risk of antibiotic-associated diarrhea. But you're more likely to develop problems if you:

• Are an older adult
• Have had surgery on your intestinal tract
• Have recently been in a hospital or nursing home
• Have a serious underlying illness such as cancer

Call your doctor right away if you experience the following signs and symptoms:

• Several episodes of loose stools or watery diarrhea for two or more consecutive days
• Fever
• Severe abdominal pain or cramping
• Pus or blood in your stool
• Lightheadedness or dizziness

These signs and symptoms may indicate a number of conditions, ranging from viral, bacterial or parasitic infections to inflammatory bowel disorders such as ulcerative colitis or Crohn's disease. In older adults with cardiovascular disease, low blood flow to the colon (ischemic colitis) also can cause these symptoms.

But if you're currently taking antibiotics or have recently finished antibiotic therapy, it's possible you have antibiotic-associated diarrhea. Your doctor can perform tests to determine the exact cause of your symptoms.

To help diagnose antibiotic-associated diarrhea, your doctor will ask about your medical history, including whether you've had recent hospitalizations or antibiotic therapy. If your symptoms are severe, you'll also likely be asked to provide samples of your stool.

These are then checked in a laboratory for the presence of C. difficile. In a certain percentage of cases, the lab test result is falsely negative. This means that although C. difficile is present in your intestinal tract, it isn't detected by the test. Repeating the test may provide a more accurate result.

Mild antibiotic-associated diarrhea isn't likely to cause any lasting problems. But pseudomembranous colitis can lead to life-threatening complications, including:

• A hole in your bowel (bowel perforation). This results from extensive damage to the lining of your large intestine. The greatest risk of a perforated bowel is that bacteria from your intestine will infect your abdominal cavity (peritonitis).
• Toxic megacolon. In this condition, your colon becomes unable to expel gas and stool, causing it to become greatly distended (megacolon). Signs and symptoms of toxic megacolon include abdominal pain and swelling, fever and weakness. You may also become groggy or disoriented. Left untreated, your colon can rupture, causing bacteria from your colon to enter your abdominal cavity. A ruptured colon requires emergency surgery and in some cases may be fatal.
• Dehydration. Severe diarrhea can lead to excessive loss of fluids and electrolytes — substances such as sodium, potassium and chloride that are essential for transmitting nerve impulses, making your muscles contract and maintaining proper levels of fluid in and around your cells. Extreme fluid loss can cause seizures and shock — a condition in which your tissues don't receive enough oxygen. Symptoms of dehydration include a very dry mouth, intense thirst, little or no urination and extreme weakness.

If you have mild diarrhea, your symptoms are likely to clear up within a few days to two weeks after your antibiotic treatment ends. In the meantime, your doctor may recommend drinking plenty of liquids to prevent dehydration and avoiding foods that may aggravate your symptoms. When diarrhea is more severe, your doctor may stop your antibiotic therapy and wait for your symptoms to subside.

In cases of very severe diarrhea, colitis or pseudomembranous colitis, you're likely to be treated with the drug metronidazole (Flagyl), which is usually taken in tablet form for 10 days. If metronidazole isn't effective, or you're pregnant or breast-feeding, you'll receive another drug, vancomycin (Vancocin).

Both metronidazole and vancomycin are antibiotics that effectively control C. difficile. Vancomycin was once the drug of choice for treating antibiotic-associated diarrhea, but it's very expensive and now usually reserved for the most resistant cases. However, vancomycin is recommended for pregnant and lactating women because the effects of metronidazole have not been proved safe in fetuses and infants.

Some people treated for pseudomembranous colitis have a recurrence of symptoms and need further treatment.

Probiotics
The term "probiotic" means "for life." It generally refers to concentrated supplements of beneficial bacteria similar to those that occur naturally in your intestinal tract. Most probiotic supplements are formulated to survive the digestive process and the highly acidic conditions in your stomach.

Probiotic supplements may help prevent antibiotic-associated diarrhea, but study results have been mixed. Some studies have shown probiotic supplements to be effective in preventing or reducing the symptoms of antibiotic-associated diarrhea, while others have not. More research is needed before probiotics can be routinely recommended for antibiotic-associated diarrhea.

Probiotic formulas are available in liquid and capsule form in drugstores and natural food stores and in some grocery stores. Many need refrigeration. In addition, commercial yogurt labeled as having live cultures contains certain "good" bacteria, such as Lactobacillus acidophilus, that may help reduce the incidence and severity of antibiotic-associated diarrhea.

Another probiotic, the yeast Saccharomyces boulardii, has been shown to help protect against pseudomembranous colitis. This yeast, taken orally, is used in Europe to help prevent symptoms of antibiotic-associated diarrhea and may also prevent recurrences of the disease.

Capsules of S. boulardii are available in natural food stores and some drugstores. Although S. boulardii rarely causes side effects, avoid it if you have a yeast allergy. And if you have HIV/AIDS, talk to your doctor before trying this product.

The following suggestions may help prevent antibiotic-associated diarrhea or reduce its severity:

• Take antibiotics only when you and your doctor feel it's absolutely necessary. Keep in mind that antibiotics won't help viral infections such as colds and flu.
• Use antibiotics exactly as prescribed. Don't increase the dose, double up on missed doses, or take the medication longer than your doctor recommends.
• Consider eating yogurt or taking probiotic capsules before, during and after antibiotic treatment.

If you develop antibiotic-associated diarrhea, colitis or pseudomembranous colitis, these dietary changes may help ease your symptoms:

• Drink plenty of fluids. Water is best, but fluids with added sodium and potassium (electrolytes) may be beneficial as well. Avoid carbonated beverages, citrus juices, alcohol and caffeinated drinks such as coffee, tea and colas, which may aggravate your symptoms.
• Emphasize soft, bland, easy-to-digest foods. These include rice, plain baked potatoes, yogurt and bananas. It's best to avoid most other fruits and their juices as well as dairy foods because they can make diarrhea worse.
• Try eating several small meals. Space meals throughout the day instead of eating two or three large ones. Smaller servings are easier to digest.
• Avoid certain foods. Stay away from spicy, fatty or fried foods and any other foods that make your symptoms worse.

Also, check with your doctor first before taking anti-diarrheal medications, which may interfere with your body's ability to eliminate toxins and lead to serious complications.