Some women may suffer from an unusual allergic reaction during or after sex—experiencing intimate discomfort, swelling or even difficulty breathing.

The cause, experts say, could be a little-known condition called seminal plasma hypersensitivity, an allergy to proteins found in semen.

Dr Michael Carroll, associate professor in reproductive science at Manchester Metropolitan University, says the condition may affect more people than previously thought—and often goes undiagnosed. 

Writing for The Conversation, he warned it is often mistaken for sexually transmitted infections (STIs), yeast infections, and general sensitivity.

But one clue you're in fact 'allergic to sex', is 'symptoms disappearing when condoms are used', he revealed.

The sex allergy is now recognised as a 'type 1 hypersensitivity', falling into the same category as peanut allergy, cat dander, and hay fever, he said.

While rare, the professor argues that is underdiagnosed due to embarrassment, stigma and a lack of awareness—meaning 'many women suffer in silence'.

In women it typically affects the vulva or vagina, but can also cause full body symptoms like hives, wheezing, dizziness, runny nose and even anaphylaxis, a potentially life-threatening immune response, he warned.

Some women may suffer from an unusual allergic reaction during or after sex—experiencing intimate discomfort, swelling or even difficulty breathing

Seminal plasma hypersensitivity was first documented in 1967, when a woman was hospitalised after a 'violent allergic reaction' to sex.

It was thought to only affect less than 100 women globally, until a study in 1997 found that nearly 12 per cent of those reporting symptoms after sex could have the allergy.

While Dr Michael Carroll had similar results in his own unpublished survey in 2013, he thinks the true figure could be higher.

The professor said: 'It’s time to bring this hidden condition out of the shadows and into the consultation room. 

'And it's not just women. It's possible some men may be allergic to their own sperm,' he added.

In men it has also been called post-orgasmic illness syndrome—because it causes symptoms that last seconds, minutes or even hours after ejaculation.

It can cause headaches, burning eyes, a runny nose, sore throat, fever, muscle weakness and fatigue, claimed French researchers last year.

More extreme symptoms include palpitations and incoherent speech, wrote an expert Hospital Center des Quatre Villes, on the outskirts of Paris in a French journal.

The researchers found that females between the ages of 20 and 59 who had sex less than once a week were at a 70 per cent increased risk of death within five years 

In the hours after climaxing, a 22-year-old man was described as suffering from sneezing, watery eyes, stomach cramps, muscle pain and deep fatigue, in a paper published in the American Journal of Case Reports earlier this year. 

While only 60 cases post-orgasmic illness syndrome have been reported since it was first discovered in 2002, it could affect many more as it is likely some don't seek medical attention.

'The problem isn't the sperm', explained Dr Michael Carroll, but rather a prostate-specific antigen, a compound found in semen.

And it isn't specific to any one partner—women can develop a reaction to any man's ejaculate. 

There was also evidence of something called 'cross-reactivity' found—when those with existing allergies can develop another one.

So if you're allergic to dogs or other things, you may have developed an allergy to sex, even if you don't realise it. 

'In one unusual case, a woman with a Brazil nut allergy broke out in hives after sex, probably due to trace nut proteins in her partner's semen,' he said.

The diagnostic process involves a review of your sexual and medical history often followed by a skin prick test with your partner's semen or blood tests. 

How much sex Americans are having per week, on average, according to a decade long study in 2018

Treatments include prophylactic antihistamines to prevent allergic reactions before exposure and anti-inflammatories.

There is also desensitisation, which involves a doctor placing a diluted semen solution in the vagina or onto the penis, at 20 minutes intervals.

While the allergy doesn't directly affect fertility—and many women with SHP successfully conceive—it can complicate things.

'Avoiding the allergen—usually the most effective treatment for allergies—isn't feasible for couples trying to conceive,' he explained.

So, if your allergic to sex and you're trying for a  baby, you may wish to choose IVF with washed sperm—to avoid the allergen PSA altogether.