Is it okay to take cephalexin if I am allergic to amoxicillin (amoxicillin)?

From the Guidelines

No, if you are allergic to amoxicillin, you should not take cephalexin without consulting a healthcare provider first. Cephalexin and amoxicillin belong to related antibiotic classes (cephalosporins and penicillins, respectively), and there is a risk of cross-reactivity between these medications, particularly if they share similar side chains 1. According to a recent study, the risk of cross-reactivity for cephalexin is around 12.9% if the patient is allergic to a penicillin with a similar side chain 1.

Key Considerations

• The severity of your previous allergic reaction to amoxicillin is important - if you experienced a severe reaction like anaphylaxis, hives, or difficulty breathing, the risk of taking cephalexin is higher.
• Cross-reactivity occurs because these antibiotics share similar chemical structures, particularly in their beta-lactam rings.
• If you need an antibiotic and have a documented amoxicillin allergy, your healthcare provider might choose an antibiotic from a completely different class, such as macrolides (like azithromycin), fluoroquinolones, or tetracyclines, depending on the type of infection being treated.

Alternatives to Cephalexin

• Macrolides (like azithromycin) are a possible alternative for patients allergic to amoxicillin 1.
• Fluoroquinolones or tetracyclines may also be considered, depending on the type of infection being treated.
• Clindamycin is another option, with a low resistance rate among GAS isolates in the United States 1.

Important Notes

• A healthcare provider should be consulted before taking any antibiotic, especially if there is a known allergy to another antibiotic.
• The risk of cross-reactivity should be carefully evaluated, and alternative antibiotics should be considered if necessary.

From the FDA Drug Label

WARNINGS BEFORE THERAPY WITH CEPHALEXIN IS INSTITUTED, CAREFUL INQUIRY SHOULD BE MADE TO DETERMINE WHETHER THE PATIENT HAS HAD PREVIOUS HYPERSENSITIVITY REACTIONS TO CEPHALEXIN, CEPHALOSPORINS, PENICILLINS, OR OTHER DRUGS IF THIS PRODUCT IS TO BE GIVEN TO PENICILLIN-SENSITIVE PATIENTS, CAUTION SHOULD BE EXERCISED BECAUSE CROSS-HYPERSENSITIVITY AMONG BETA-LACTAM ANTIBIOTICS HAS BEEN CLEARLY DOCUMENTED AND MAY OCCUR IN UP TO 10% OF PATIENTS WITH A HISTORY OF PENICILLIN ALLERGY.

It's not okay to take cephalexin if you are allergic to amoxicillin without caution. There is a risk of cross-hypersensitivity among beta-lactam antibiotics, including penicillins and cephalosporins, which may occur in up to 10% of patients with a history of penicillin allergy 2.

• Key points:
  • Cephalexin should be given with caution to penicillin-sensitive patients.
  • Cross-hypersensitivity among beta-lactam antibiotics has been documented.
  • Patients with a history of penicillin allergy are at risk of an allergic reaction to cephalexin.

From the Research

Allergy Considerations

• If you are allergic to amoxicillin, it's essential to exercise caution when considering other antibiotics, including cephalexin.
• The cross-reactivity between penicillins (such as amoxicillin) and cephalosporins (such as cephalexin) is lower than previously reported, with an overall cross-reactivity rate of approximately 1% when using first-generation cephalosporins or cephalosporins with similar R1 side chains 3.
• Cephalexin is a first-generation cephalosporin, and its use in patients with a penicillin allergy carries a negligible risk of cross-allergy, especially if the penicillin allergy is not severe 3, 4.

Safety of Cephalexin

• Studies suggest that cephalexin can be prescribed safely for penicillin-allergic patients, with a low risk of cross-reactivity 3, 4.
• However, it's crucial to note that the risk of cross-reactivity may be higher in patients with a severe penicillin allergy, such as those who have experienced anaphylaxis or severe cutaneous adverse reactions 5.
• If you have a confirmed penicillin allergy, your healthcare provider may recommend alternative antibiotics or perform further testing to determine the safety of cephalexin use 6, 7.

Key Takeaways

• The decision to use cephalexin in patients with a penicillin allergy should be made on a case-by-case basis, taking into account the severity of the allergy and the potential benefits and risks of treatment 3, 5.
• If you are allergic to amoxicillin and are considering cephalexin, consult your healthcare provider to discuss the potential risks and benefits and determine the best course of treatment 3, 4.