It's commonly thought that if you have an allergy to penicillin or penicillin related products, you have a high risk of have been allergic to the cephalosporin class of antibiotics, which includes cefdinir. This belief is held for multiple reasons, not limited to the fact that both penicillin antibiotics and cephalosporin antibiotics are very closely related structurally.

Adding to this belief, studies completed in the 1960s and 1970's showed that penicillin and cephalosporin "cross-reactivity" was as high as 50%. However, current analysis shows that this high rate was most likely due to penicillin contaminated products and other factors. The true "cross-reactivity" rate is far lower.

Current data suggests that the overall cross-reactivity rate between penicillin antibiotics and cephalosporin antibiotics is about 3%. More specifically, the rate of cross-reactivity is:

  • About 0.1% in individuals without a skin test confirmed penicillin allergy.
  • About 0.1% in individuals who have had mild reactions to penicillin.
  • About 2% in individuals with a confirmed positive penicillin skin test. 

We now know that the chance of "cross-reactivity" in individuals is highest with first generation cephalosporins. These include:

  • Keflex (cephalexin)
  • Duricef (cefadroxil)
  • Ancef (cefazolin)

Generally, in those who had a mild penicillin reaction, most cephalosporins can be tried. If you had a more severe reaction to penicillin, caution is generally needed and 1st generation cephalosporins are avoided in most cases. Most 2nd, 3rd and 4th generation cephalosporins can be tried under certain conditions.

It is often necessary to determine the type of reaction you had to penicillin to aide in the decision of which cephalosporin is safe to try. There are multiple types of allergic reactions:

Immediate/Accelerated Reactions (Type I)

These reactions happen quickly and are caused by penicillin specific antibodies. They generally occur within one hour to 72 hours of taking penicillin. Symptoms of this type of allergic reaction include:

  • Swelling
  • Low blood pressure
  • Hives
  • Difficulty breathing.

Late/Delayed & Idiopathic Reactions (Type II, III, IV, V)

These types of reactions typically occur more than 72 hours after dosing. The symptoms can vary from greatly in severity and seriousness, from a simple rash to life-threatening swelling and organ failure.

Most clinicians will not use a first generation cephalosporin in any patient that had an immediate allergic reaction (Type I) to penicillin. The later generation cephalosporins are sometimes used, depending on the situation. Sometimes an individual will be "challenged" on a low dose of a cephalosporin to determine if it will be tolerated.

Cefdinir is a 3rd generation cephalosporin. It is generally considered okay to be utilized in those with a mild reactions to penicillin. It can also be used in some cases in those with a more severe penicillin allergy. Generally a small "test" dose can be given to determine if an allergic reaction will occur before administering a full dose.


  • Overall, about 2 to 3% of patients who have experienced a reaction to penicillin will react to a cephalosporin. If the initial reaction was mild, this percentage improves significantly.
  • 1st generation cephalosporins have a higher chance of cross-reactivity in those with a penicillin allergy. However, the chances are small if the previous reaction to penicillin was minor.
  • With individuals who had more serious reactions to penicillin, later generation cephalosporins are sometimes utilized after allergy testing and a "test" dose.
  • If a reaction were to occur, it may happen immediately (Type 1 reactions) or days later (Type II,II,IV).