Antibiotics typically do not cause serious side effects. Some people may experience symptoms of minor discomfort, such as upset stomach. A person’s rate of digestion may be affected by antibiotics, causing food to leave the stomach and enter the intestines too early and increasing intestinal contractions, both of which can contribute to diarrhea. Antibiotic use can also increase a person’s photosensitivity (likelihood of sunburn when exposed to sunlight).

Vaginal yeast infections (candidiasis) are a common side effect of antibiotic use in women. By reducing bacteria levels, antibiotics can cause an overgrowth of other organisms in the body, such as yeast.

Yeast infections are treatable with both prescription and over-the-counter medications. These types of infections may also be avoided by the daily consumption of “live culture” yogurt (available at most grocery stores) while undergoing antibiotic treatment.

Some side effects associated with antibiotic use may be severe. Patients who take antibiotics may experience antibiotic-associated diarrhea (AAD). This occurs when antibiotics disrupt the natural bacterial balance in the body by killing too many good “good” bacteria, allowing “bad” bacteria to flourish in the intestinal tract. As the number of bad bacteria increase, the patient may experience frequent, watery bowel movements. Usually, a mild case of AAD begins about five to 10 days after starting antibiotic therapy. However, in some cases, AAD may be more severe. Such episodes can lead to inflammation of the large intestine (antibiotic-associated colitis) that causes abdominal pain, bloody diarrhea and fever.

A more serious version of this illness (pseudomembranous colitis) is potentially deadly. Concentrated supplements of beneficial bacteria (e.g., lactobacillus acidophilus) or consuming yogurt can sometimes help prevent or relieve symptoms associated with AAD.

Adverse reactions to antibiotics may be more serious in patients with weakened immune systems (e.g., due to HIV & AIDS). Some patients may also experience an allergic reaction, including a skin rash with or without itching. Severe allergic reactions include swelling, hives, shortness of breath and collapse (anaphylactic shock).

Patients who experience serious side effects associated with antibiotic use should seek medical attention. These side effects include:

• Hives
• Skin rash
• Fever
• Nausea
• Dark or discolored tongue
• Dizziness or lightheadedness
• Abnormally fast heartbeat (tachycardia)
• Unusual fatigue or weakness
• Headache
• Ringing in ears or deafness
• Bloating
• Unusual thirst
• Loss of appetite
• Weight loss
• Unusual bleeding or bruising
• Confusion
• Back or joint pain or stiffness
• Jaundice (yellow eyes and/or skin)
• Itchiness of armpits, groin or rectum
• Muscle twitches or spasms
• Convulsions or seizures

Other possible but relatively rare complications include kidney failure and liver abnormalities.