Cefaclor XR Side Effects: What to Expect and When to Call Your Doctor

Like all antibiotics, Cefaclor XR can cause side effects. Most are mild and manageable, but some require immediate medical attention. Knowing the difference can save you from unnecessary anxiety — or from ignoring a serious warning sign. Here's what you need to know.

Common Side Effects of Cefaclor XR (Usually Mild)

These side effects are relatively common and often resolve on their own as your body adjusts to the medication. They generally do not require stopping the antibiotic, but you should mention them to your doctor if they're severe or persistent:

Diarrhea: The most common GI side effect; mild loose stools are normal. If diarrhea becomes severe, watery, bloody, or accompanied by cramps or fever, stop the medication and call your doctor immediately (see serious side effects below).

Nausea and stomach upset: Taking Cefaclor XR with food (which is required for proper absorption) can help reduce nausea.

Vomiting: Occasional nausea and vomiting can occur, especially early in treatment.

Headache: Mild headaches have been reported with cephalosporin antibiotics.

Mild rash: A mild skin rash may appear. Note: Any rash should be reported to your doctor — a mild rash can sometimes be a precursor to a more serious allergic reaction.

Vaginal yeast infection: Antibiotics kill some normal bacteria in the body, which can allow yeast to overgrow. This is more common in women taking multiple courses of antibiotics.

Serious Side Effects — Call Your Doctor Immediately

Stop taking Cefaclor XR and seek emergency medical help or call your doctor right away if you experience any of the following:

Severe allergic reaction (anaphylaxis): Hives, difficulty breathing, swelling of the face, lips, tongue, or throat, rapid heartbeat, dizziness. This is a medical emergency — call 911.

C. difficile-associated diarrhea (CDAD): Watery or bloody diarrhea, severe stomach cramps, fever, nausea, or frequent bowel movements (even up to 2 months after stopping the antibiotic). CDAD is a serious infection caused by antibiotic disruption of gut bacteria.

Stevens-Johnson Syndrome (SJS) / Toxic Epidermal Necrolysis (TEN): Painful red or purple skin rash that blisters and peels, mouth sores, eye inflammation. These rare but life-threatening skin reactions can progress rapidly.

Serum sickness-like reaction: Cefaclor is more likely than other cephalosporins to cause serum sickness-like reactions: skin rash, joint pain (arthralgia), and fever appearing several days into treatment. This is not a true serum sickness but requires prompt evaluation.

Seizures: Some people who have taken cefaclor have reported seizures, especially at high doses. This risk is higher in patients with kidney disease who receive inappropriately high doses.

Signs of liver problems: Yellowing of the skin or eyes (jaundice), dark urine, severe fatigue.

The Serum Sickness Warning: Unique to Cefaclor

One side effect worth special attention: cefaclor (the active ingredient in Cefaclor XR) is associated with a higher rate of serum sickness-like reactions than other cephalosporins. This reaction typically appears 5-7 days into treatment and involves a combination of skin rash, joint pain, and fever. It can be mistaken for a worsening infection. If you notice these symptoms, call your doctor — do not dismiss them.

How to Reduce Common Side Effects

Always take with food. This is required for proper absorption AND can help reduce nausea and stomach upset.

Don't crush or chew the tablet. Swallow the extended-release tablet whole to maintain proper drug release and reduce the risk of concentrated GI exposure.

Consider probiotics. Some research supports the use of Lactobacillus-containing probiotics during and after antibiotic treatment to reduce diarrhea. Discuss with your pharmacist or doctor.

Stay hydrated. If you experience mild diarrhea, drink plenty of fluids to avoid dehydration.

False Positive Urine Glucose Test

Cefaclor can cause a false-positive result on certain urine glucose tests (Clinitest copper reduction tests). If you have diabetes and test your urine for sugar, use Clinistix or TesTape instead — these enzyme-based tests are not affected by cefaclor.

Also see our full guide to Cefaclor XR Drug Interactions to understand which other medications can increase your risk of side effects.