The 76-year-old, who has not been named, inadvertently swallowed the felt-tip in early 1986 while she was attempting to check her tonsils.
As she held down her tongue it fell down her throat and she rushed to hospital to have it removed.
But her husband and doctors dismissed her story after scans and X-rays failed to find anything. She later left the hospital and forgot about it.
More than 25 years later she was seen by a gastrointestinal specialist at the Royal Devon and Exeter Hospital Foundation Trust after suffering stomach pains, weight loss and diarrhoea.
An urgent investigation disclosed that areas of the woman’s large intestine had become inflamed, which was causing the abdominal pain.
A CAT scan of her abdomen appeared to uncover a “linear foreign body in the stomach” that looked remarkably similar to a pen.
The plastic felt-tip was later successfully removed during surgery under general anaesthetic but doctors were astounded when they removed the pen undamaged and still in full working order.
As a joke they wrote the woman a note simply saying “hello”. It also had not left any internal damage to the OAP.
Medics later discovered her symptoms were caused by severe diverticulitis a common condition among older people that causes small pouches to bulge from the colon.
The details emerged in report from doctors at the hospital’s gastroenterology department, which was submitted to the British Medical Journal.
”On subsequent questioning, she recalled unintentionally swallowing a pen 25 years earlier,” wrote authors Drs Oliver Richard Waters, Tawfique Daneshmend and Tarek Shirazi.
”While she was interrogating a spot on her tonsil with the pen she slipped, fell and swallowed the pen by mistake.
”Her husband and general practitioner dismissed her story and plain abdominal films done at the time were reported as normal.”
Reporting the case in the BMJ’s Case Reports, the team added: ”It was subsequently removed in a combined endoscopic and ear, nose and throat procedure under general anaesthetic. The pen was still in working order.
”This case highlights that plain abdominal X-rays may not identify ingested plastic objects and occasionally it may be worth believing the patient’s account however unlikely it may be.”
Neither a spokesman or the doctors were available for comment on Wednesday.