Cefazolin Side Effects: What You Need to Know

Cefazolin is a widely used injectable antibiotic that fights bacterial infections effectively. Like all antibiotics, it can cause side effects — most of them mild and temporary. But some side effects are serious and need immediate medical attention.

This guide covers the most common Cefazolin side effects, the warning signs you shouldn't ignore, and practical tips for managing discomfort during treatment.

What Is Cefazolin?

Cefazolin (formerly sold under brand names Ancef and Kefzol) is a first-generation cephalosporin antibiotic. It's given by IV or intramuscular injection to treat a wide range of infections, including skin infections, urinary tract infections, bone and joint infections, bloodstream infections, and heart valve infections (endocarditis). It's also one of the most commonly used antibiotics for surgical infection prevention.

To learn more about how this medication works in your body, see our guide on how Cefazolin works. For a broader overview, check out What Is Cefazolin?

Common Side Effects

Most people tolerate Cefazolin well. The most common side effects are mild and often go away on their own:

• Diarrhea — Loose stools are common with most antibiotics, including Cefazolin. It happens because the drug can disrupt the normal bacteria in your gut.
• Nausea and vomiting — Some people feel queasy, especially shortly after receiving a dose.
• Injection site reactions — Pain, redness, or swelling where the IV or IM injection is given. This is one of the most frequently reported side effects.
• Oral thrush — White patches in the mouth caused by yeast overgrowth. This can happen when antibiotics kill off normal bacteria that keep yeast in check.
• Vaginal yeast infection — Similar to oral thrush, some women develop a vaginal yeast infection during or after Cefazolin treatment.

These side effects are generally manageable and don't require stopping the medication. Talk to your nurse or doctor if they bother you — there are often simple solutions.

Serious Side Effects — When to Call Your Doctor

While rare, some Cefazolin side effects are serious. Contact your doctor immediately or go to the emergency room if you experience any of the following:

• Severe allergic reaction — Hives, rash, swelling of the face or throat, difficulty breathing, or feeling faint. Anaphylaxis is a medical emergency.
• Clostridioides difficile (C. diff) infection — Watery diarrhea that won't stop, bloody stools, stomach cramps, or fever. C. diff can occur during or even weeks after finishing antibiotics.
• Severe skin reactions — Blistering, peeling skin, or a widespread rash. In rare cases, this could indicate Stevens-Johnson syndrome.
• Seizures — Especially in patients with kidney problems or those receiving high doses.
• Kidney problems — Decreased urination, swelling in the legs or feet, or unusual fatigue.
• Liver problems — Dark urine, yellowing of the skin or eyes (jaundice), or severe stomach pain.
• Signs of hemolytic anemia — Unusual tiredness, pale skin, rapid heartbeat, or dark-colored urine.

Important: Do not wait to see if serious symptoms get better on their own. Early treatment makes a big difference.

Side Effects in Specific Populations

Pregnancy and Breastfeeding

Cefazolin is classified as Pregnancy Category B, meaning it's considered relatively safe during pregnancy. It's commonly used for surgical prophylaxis during cesarean sections and for Group B strep prevention during labor. Small amounts pass into breast milk, but it's generally considered compatible with breastfeeding.

Kidney Disease

Patients with reduced kidney function need dose adjustments because Cefazolin is cleared through the kidneys. Higher-than-normal drug levels can increase the risk of seizures and other toxicity. Your doctor will check your kidney function and adjust your dose accordingly.

Penicillin Allergy

If you have a penicillin allergy, there's a small chance (about 1-2%) of cross-reactivity with Cefazolin. However, if your penicillin reaction was mild (like a rash), your doctor may still consider Cefazolin safe for you. If you've had a severe reaction (anaphylaxis) to penicillin, your doctor will likely choose a different antibiotic.

Children

Cefazolin is safe for pediatric patients, with dosing based on body weight. Side effects in children are similar to adults.

How to Manage Cefazolin Side Effects

Here are practical tips for dealing with the most common side effects:

• For diarrhea: Stay hydrated with water and clear fluids. Eat bland foods like rice, bananas, and toast. Avoid dairy until symptoms improve. Do NOT take anti-diarrheal medication without asking your doctor — it can make C. diff worse.
• For nausea: Eat small, frequent meals. Ginger tea or crackers before your infusion may help.
• For injection site pain: Apply a cool compress. Let your nurse know if the IV site becomes very red, swollen, or warm — this could indicate phlebitis (vein inflammation).
• For yeast infections: Over-the-counter antifungal treatments usually work well. Ask your doctor before starting any new medication.
• For oral thrush: Your doctor may prescribe an antifungal mouth rinse. Good oral hygiene helps prevent it.

For information on what drugs to avoid while taking Cefazolin, see our guide on Cefazolin drug interactions.

Final Thoughts

Cefazolin is a well-established antibiotic with a strong safety record. Most side effects are mild — things like diarrhea, nausea, and injection site discomfort. Serious reactions are rare but important to recognize.

The key is knowing what to watch for and when to get help. If something feels wrong during your treatment, don't hesitate to contact your healthcare provider.

If you're having trouble finding Cefazolin due to supply issues, Medfinder can help you locate it nearby. You can also read about the current Cefazolin shortage and available alternatives.