Can cephalexin cause hives? Understanding the Allergic Reaction

October 11, 2025 •

3 min read

Adverse reactions to cephalexin, a cephalosporin antibiotic, occur in about 0.5%–2% of patients. Skin reactions, such as hives, may indicate an immune-mediated allergic response.

Quick Summary

Cephalexin can cause hives, an allergic reaction symptom. Reactions range from mild rashes to severe anaphylaxis. Recognize the signs of an allergy and seek medical advice promptly. Cross-reactivity with penicillin is also a risk.

Key Points

Allergic Reactions: Cephalexin can cause hives, an allergic reaction, which is an immune-mediated response to the drug.
Mild vs. Severe Symptoms: Hives are a sign of a mild reaction, but facial swelling or breathing difficulties indicate severe, life-threatening anaphylaxis, requiring immediate emergency care.
Link to Penicillin: There is a risk of cross-reactivity between cephalexin and penicillin; report a penicillin allergy to a doctor.
Immediate Action: If hives or other allergic symptoms develop, stop taking the medication and contact your doctor immediately.
Emergency Response: For anaphylaxis symptoms like swelling and breathing difficulty, call 911 immediately.
Management and Alternatives: Antihistamines may help mild symptoms under a doctor's guidance; alternative antibiotics from a different drug class may be necessary.

Can Cephalexin Cause Hives?

Yes, cephalexin, known by brand names like Keflex, can cause hives. Hives, or urticaria, are raised, itchy welts on the skin that result from an allergic reaction. When someone is allergic to a drug, the immune system identifies the medication as harmful. This triggers the release of histamines and other chemicals, leading to hives and swelling. Allergic reactions to cephalexin vary in severity, ranging from mild to, in rare cases, severe and life-threatening.

Symptoms of a Cephalexin Allergic Reaction

Recognizing the symptoms of an allergic reaction is crucial. Some reactions are mild and manageable, while others require immediate medical care.

Mild symptoms often include:

Hives: Itchy, red, or skin-colored welts.
Rash: A non-hives skin eruption.
Itching: General pruritus without a visible rash.

Severe symptoms, known as anaphylaxis, can include:

Swelling: Of the face, lips, tongue, or throat (angioedema).
Trouble breathing or swallowing.
Wheezing.
Tightness in the chest or throat.
Extreme fatigue or dizziness.
Anaphylaxis is a medical emergency; call 911 immediately.

The Connection to Penicillin Allergy

Cephalexin is a cephalosporin antibiotic, structurally similar to penicillins. This similarity means some with a penicillin allergy may also be allergic to cephalexin, known as cross-reactivity. The estimated cross-reactivity rate was once as high as 10%, but recent data suggests the risk is lower for many, particularly those with non-severe, delayed-type allergies. Always inform a doctor about any history of drug allergies, especially to penicillin, before starting cephalexin.

What to Do If You Get Hives from Cephalexin

If hives or other allergic reaction signs appear while taking cephalexin, stop the medication and contact a doctor immediately. They can assess symptoms and decide on a course of action, possibly including an alternative antibiotic.

Action steps:

Stop the medication. Do not continue cephalexin if an allergic reaction is suspected.
Contact your doctor or pharmacist. Report symptoms immediately. For mild reactions, an over-the-counter antihistamine may be recommended to manage itching.
Seek emergency medical help for severe symptoms. If swelling, difficulty breathing, or throat tightness occur, call 911 or go to the nearest emergency room immediately.
Inform all healthcare providers. Ensure the allergy is documented in medical records to prevent future exposure.

Other Potential Side Effects of Cephalexin

Distinguish between an allergic reaction and other non-allergic side effects. Hives signal an allergy, while other common side effects may include:

Nausea and vomiting
Diarrhea, which can be severe (Clostridioides difficile-associated diarrhea)
Stomach pain
Headache
Genital or rectal itching

These side effects, though unpleasant, are not an allergic reaction. Discuss any persistent or severe side effects with a doctor.

Comparison of Allergic Reactions to Cephalexin


Feature	Mild Allergic Reaction	Severe Allergic Reaction (Anaphylaxis)
Onset	Minutes to days after a dose, sometimes weeks later.	Typically rapid, within minutes to an hour of exposure.
Symptoms	Hives, itchy rash, general itching.	Swelling of the face, tongue, or throat; difficulty breathing; wheezing; chest tightness; rapid heart rate; sudden drop in blood pressure.
Mechanism	Immune system response releasing inflammatory mediators, usually less severe.	Widespread, rapid release of histamines and other chemicals, causing systemic effects.
Action Required	Stop medication and contact your prescribing physician for advice and possible alternative treatment.	Call 911 or go to an emergency room immediately.
Management	Antihistamines may be recommended for symptom relief.	Epinephrine (like an EpiPen) is the primary treatment, along with other supportive care in a hospital setting.

Conclusion

In conclusion, the answer to "Can cephalexin cause hives?" is yes. Hives are a common sign of an allergic reaction to this antibiotic and, though often mild, should be addressed. Awareness of mild and severe allergy symptoms is critical for patient safety. Always inform your healthcare provider of any drug allergies, especially penicillin allergies, before taking cephalexin due to cross-reactivity. Stopping the medication and seeking medical advice when allergic symptoms appear is the best action. For detailed drug information, refer to a reliable source such as the MedlinePlus Drug Information on Cephalexin.

Frequently Asked Questions

Allergies to cephalosporin antibiotics, like cephalexin, are reported in a small percentage of patients, with adverse reactions documented in approximately 0.5%–2% of those newly exposed. However, the incidence of true allergic reactions is lower than the rate of self-reported allergies.

Hives from cephalexin can appear soon after starting the medication, or sometimes develop days or even weeks after beginning treatment. Delayed skin reactions can occur weeks or months after starting the antibiotic.

A side effect is a common and often predictable reaction to a medication, such as nausea or diarrhea. An allergy is an immune system-mediated response to the drug, which is unpredictable and can cause symptoms like hives, swelling, or breathing difficulties.

Due to the risk of cross-reactivity between penicillin and cephalosporin antibiotics like cephalexin, taking cephalexin is not recommended if there is a known penicillin allergy, especially if it was a severe reaction. Always inform your doctor about your complete drug allergy history.

If allergic to cephalexin, your doctor may prescribe an alternative antibiotic from a different drug class. Options include azithromycin, doxycycline, or certain fluoroquinolones, depending on the infection being treated.

For mild hives, a doctor may recommend an antihistamine to help decrease the itching and rash. However, the primary action is to stop taking cephalexin. In severe cases of anaphylaxis, emergency treatment with epinephrine and supportive care is necessary.

The severity of an allergic reaction is not always predictable, but it can worsen with subsequent exposures. Because of this unpredictability, any allergic reaction, even mild hives, should be reported to a doctor immediately, and cephalexin should not be taken again.