To medical experts, it's known as coital cephalgia, an intense, searing headache that's brought on by sexual activity. But many people know it as an extreme case of, "not tonight dear, I have a headache."

For Will Ashton, 31, it was one of the most terrifying moments of his life, and it occurred while he was having sex with his girlfriend. "It was like a sledgehammer had smacked me in the back of my head," Ashton told the Daily Mail. "I collapsed on the bed, groaning and in shock. I couldn't open my eyes, and I felt dazed for about 30 seconds."

When his headache was still throbbing two days later, he visited a doctor who diagnosed him with coital cephalgia, Also known as benign sex headache (BSH) or "coital thunderclap headache," according to the British Journal of Medical Practitioners, the condition strikes an estimated one in 100 people.

Researchers believe, however, that the condition may be more common, since patients are often too embarrassed to discuss it with their doctors. And the condition isn't well understood even by medical experts. Severe headaches related to sexual activity have also been reported by teens and by people watching pornography.

Men are three to four times more likely to be diagnosed with coital cephalgia, and the headaches are more likely to be experienced by those who also suffer from migraines, according to the British Journal of Medical Practitioners. While some people experience a "pre-orgasmic headache" that slowly builds up, others describe a sudden splitting headache at or near the moment of orgasm.

An attack of coital cephalgia isn't like an ordinary headache: It's often a sharp, piercing pain that begins at the base of the skull and moves up toward the front of the head and behind the eyes, according to the British Journal of Medical Practitioners. The attacks can occur off and on for months or years, and they often go away on their own.

Doctors stress it's important to immediately seek medical attention in the event of any sudden, powerful headache. An aneurysm, brain tumor, stroke, spinal disease or cerebral hemorrhage can also cause a severe headache and must be ruled out before coital cephalgia can be accurately diagnosed, according to the British Journal of Medical Practitioners.

Fortunately, there are medical treatments for the condition: Indomethacin (Indocin), a nonsteroidal anti-inflammatory drug, has been used successfully to treat coital cephalgia. Other options include propranolol hydrochloride (Inderal) and naratriptan (Amerge); all these drugs work best when taken prior to sexual activity, studies have shown.

Now married, Ashton was prescribed indomethacin and hasn't experienced a sexual headache for months, though "having to take a pill before sex does kind of kill the spontaneity," he told the Daily Mail.

Follow LiveScience on Twitter @livescience. We're also on Facebook & Google+.