Are you forgoing penicillin during pregnancy because of an allergy? Not so fast, say researchers

In a recent systematic review, researchers at the University of Toronto and Sinai Health have determined that nearly 95 per cent of pregnant patients who believe they’re allergic to penicillin — the most commonly used antibiotic in obstetrics — are not actually allergic. These people can safely and effectively undergo an allergy evaluation during pregnancy to have that label removed.

“About one in ten people will report a penicillin allergy. But of those, only one in twenty actually has symptoms consistent with having an allergy,” says Lindsay Reddeman, a fifth-year resident at in the ob-gyn department.

The findings were published in the American Journal of Obstetrics and Gynecology in January 2026. In addition to lead author Reddeman, the research team includes several members of Temerty Medicine’s department of obstetrics and gynaecology: assistant professor and senior author Kristin Harris; professor and chair Kellie Murphy; and Justin Lim, a fourth-year resident.

A penicillin allergy is not trivial. Penicillin, which is in the class of antibiotics called beta-lactams, is considered the first-line treatment for many infections in pregnancy.

“That includes urinary tract infections, chest infections, inflammation of the fetal membranes, certain sexually transmitted infections, which in some cases cannot be treated effectively without penicillin,” says Reddeman. “We also use penicillin and other beta-lactam antibiotics for preventive measures during caesarian section or operative birth.”

Upwards of 40 per cent of pregnant patients will require an antibiotic at some point during their pregnancy. For patients who report an allergy, “it means that they get second-line treatments and suboptimal targeting of suspected pathogens, which can contribute to worsening infections and antibiotic resistance,” says Reddeman.

The poor outcomes aren’t restricted to the mother. Studies have shown that nonpenicillin alternatives can have higher rates of bacterial resistance, which may be associated with increased rates of early onset Group B streptococcus infection in the newborn. And in utero exposure to the broad-spectrum antibiotics often used in place of penicillin has been associated with development of asthma and allergies in childhood.

With these considerations, it’s in the best interest of both the patient and the physician to verify whether a purported allergy is true; the process of removing the allergy label is referred to as delabelling.

The good news is that antenatal penicillin allergy evaluation is simple and fast.

“Based on the findings of our study, we know that the intervention is safe, that it is effective — that we get the right answer — and that pregnant patients are willing to do the testing,” says Harris, who supervised Reddeman on the project.

The team believes that increasing access to allergy evaluation will accelerate delabelling efforts. One possible solution is designated evaluation clinics in environments, such as hospitals, that have the trained personnel and available medications to safeguard against the rare possibility of an adverse reaction.

Last April, Harris received funding from the AMO-Innovation Fund at Sinai Health and University Health Network to model such a clinic. It launched on January 28, 2026, and will run one half-day a month at Mount Sinai Hospital.

Patients are considered candidates for referral after achieving a certain risk score on the Penicillin Allergy Decision Rule (PEN-FAST), which is a widely recognized grading tool for penicillin allergy testing.

“Initially, the patient will do a skin-prick test,” explains Harris. “If that is non-reactive, Dr. Fahmy will give them a small dose of oral antibiotic. Then they sit for about an hour, under observation for signs of reaction. If everything looks good, they will be delabelled on that day.”

Reddeman appreciates that pregnancy is a time where patients are cautious to undergo additional procedures.

“Many patients have been carrying this label for a very long time, and they may not know the full history of why it was attached to them,” she says. “The most common story is that ‘My mom told me I got a rash as a baby after she gave me penicillin.’ Often, this patient had the rash in response to whatever virus or infection was in their body.”

Even in the case of true allergy, Reddeman adds, “we know that 80 per cent of those allergies wane and resolve within ten years from the first observed reaction.”

In their review, the researchers found that when providers take the time to discuss with their patients the benefits and the safety of penicillin allergy evaluation, over 60 per cent are willing to undergo the evaluation.

“Pregnancy is a good time to approach otherwise healthy people because it’s likely the most involved they’ve been with the health-care system since childhood, and they’re very motivated to change,” says Harris.

Ideally, the newly launched clinic becomes a success story that’s replicated in other sites. And Harris and Reddeman envision other opportunities to delabel this patient population: for example, when they’re admitted to hospital for other reasons, or when they’re undergoing routine check-ups.

To Reddeman, antenatal penicillin allergy evaluation is a no-brainer.

“As I was working on this paper, I kept thinking to myself, ‘This is not fancy science. This is not a fancy intervention. This is really obvious,’” she says. “And it would make a real difference for patients.”