A case report published in the European Medical Journal documents a rare and unusual allergic reaction believed to have been triggered by oral exposure to semen containing trace amounts of an antibiotic. The authors emphasize that allergic reactions connected with sexual activity are likely underreported and poorly understood, observing that:
Allergic reactions associated with intimate behaviours are markedly underdiagnosed and rarely reported, such that patients may not reveal or suspect a relationship between their allergic reactions and sexual activity.
The case involved a 37-year-old woman with a history of allergic rhinoconjunctivitis, well-controlled mild intermittent asthma, and a penicillin allergy that had caused hives more than 15 years earlier. About 10 minutes after performing oral sex and swallowing her husband’s semen, she developed itching in her mouth, wheezing, nausea, anxiety, and flushing of her face and chest. She took 50 mg of oral diphenhydramine (Benadryl), but her symptoms continued to worsen, peaking about 50 minutes after the encounter before gradually resolving over the following hour.
The reaction puzzled doctors because the couple had regularly engaged in oral sex without any previous problems. Further questioning revealed one important difference: her husband had recently been taking cephalexin, a commonly prescribed first-generation cephalosporin antibiotic. This led allergists to suspect that she had been indirectly exposed to the medication through his semen. As the authors explained, “The anaphylaxis reaction presented here was believed to be associated with oral exposure to semen from a partner receiving cephalexin.”
Doctors conducted additional testing to determine what might have triggered the reaction. Skin testing showed that the woman remained sensitive to penicillin, while tests using her husband’s medication-free seminal plasma and the related antibiotic cefazolin were negative. Cephalexin itself could not be used for standard skin testing because it is available only in oral form, and the authors cautioned that a negative cefazolin test could not rule out an allergy to cephalexin. The husband then agreed to take cephalexin again at 500 mg four times daily for two days so researchers could test his blood, urine, and semen. After treatment, cephalexin concentrations reached 2,400 µg/mL in his urine and 12 µg/mL in his blood, but levels in his seminal plasma remained below the laboratory test’s detection limit of 6.25 µg/mL.
The researchers proposed two possible explanations for how the woman could have been exposed despite the drug not being detected in the semen. One possibility was that a tiny amount of urine, which contained a very high concentration of cephalexin, mixed with the ejaculate. The report notes that “assuming the volume of 1 mL has 20 small drops, a single drop would contain approximately 120 µg of the drug.” Another possibility is that cephalexin was present in the semen at levels too low for the laboratory test to detect but still high enough to trigger a reaction. The woman’s existing sensitivity to penicillin may also have played a role through cross-reactivity with cephalexin, although that was not proven. The authors note that cephalexin’s low ability to dissolve in fat, combined with the lipid-rich and alkaline nature of semen, may help explain why so little of the drug would be present there.
Proving the cause beyond doubt would have required deliberately exposing the woman to her husband’s semen after he had taken cephalexin—something doctors considered too dangerous. The medical team explained that “this could carry a high risk that would not be acceptable to the authors, the patient, or the institutional Ethics Review Board, particularly since it is not a part of the standard of care in routine clinical practice.” As a result, the connection between cephalexin and the woman’s reaction remains the most likely explanation proposed by the authors rather than a definitively proven cause. They also noted that, to their knowledge, this may be the first reported case of a systemic drug-allergic reaction in a woman resulting from oral sex.
To prevent another reaction, doctors advised the couple to take precautions whenever the husband uses certain antibiotics: “Whenever her husband is taking penicillin or a first-generation cephalosporin, the couple should avoid fellatio and use barrier protection for vaginal intercourse upon initiation of the antibiotic and continuing for a few days after its discontinuation.” The case highlights an unusual but potentially important route of allergen exposure and the need for doctors and patients to consider sexual activity when the cause of an allergic reaction isn’t obvious. As the authors conclude, “whenever the cause of an acute allergic reaction is not obvious, the differential diagnosis should include the potential route of oral sex that warrants greater clinical awareness.”
