An expert has explained an unexpected side effect being experienced by people who regularly take painkillers, including paracetamol, codeine and ibuprofen. Dan Baumgardt, senior lecturer at the University of Bristol's school of neuroscience says doctors often see the symptom.

He said that, while people take over-the-counter and prescription drugs for pain, if they are taken for more than three months, then they can cause persistent headaches. Dan said: "Doctors often see another type of persistent headache that has a clear pattern. Patients report getting repeated headaches that started or got worse after taking painkillers regularly for three months or longer.

"This can happen in people with migraines, tension headache, or other painful conditions like back or joint pain. Some may take several types of medication, often more and more frequently, and end up stuck in a frustrating cycle that doesn’t seem to make sense at first. The probable diagnosis is medication-overuse headaches. This condition is thought to affect about 1–2% of people and is three-to-four times more common in women."

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He told The Conversation: "The culprit is often the painkillers themselves. Opiates like codeine, used to treat moderate pain from injuries or after surgery, come with a long list of side-effects including constipation, drowsiness, nausea, hallucinations – and headaches.

"It’s not just strong opiate-based medications that can cause headaches. Common painkillers like paracetamol and NSAIDs (non-steroidal anti-inflammatories, such as ibuprofen) can also play a role. Some medications even combine paracetamol with an opiate, such as co-codamol.

"Paracetamol has a simpler side-effect profile compared with drugs like codeine. When taken within the recommended daily limits – which depend on age and weight – it is generally a safe and effective painkiller. This has contributed to its widespread use and easy availability.

"However, taking more than the recommended dose or using it too often can be very dangerous. This can lead to serious – sometimes fatal – complications, such as liver failure. Even though side-effects are less common, studies have shown that regular use of paracetamol alone can also trigger chronic headaches in some people.

"Other drugs besides painkillers can also cause problems. Using triptans too often – medications to stop migraine attacks – can also lead to medication-overuse headaches."

Dan said the term “overuse” might make it sound like patients are taking more than the recommended daily dose, but in many cases patients are neither exceeding dose limits nor taking the medication every single day. For paracetamol or NSAIDs, medication-overuse headaches may develop if they are taken on 15 or more days per month.

With opiates, headaches can appear with even less frequent use – sometimes after just ten days a month.

Dan said: "I t’s important to talk to a doctor if you need to use any painkiller, even over-the-counter ones, for a long time. Not everyone will develop medication-overuse headaches, and the risk seems to differ from person to person, meaning individual susceptibility plays a big role."

Treatment involves gradually stopping the medication under guidance, eventually stopping it completely. Dan said: " If you’re having headaches on more than 15 days a month, it’s important to see your GP. Talking it through can help identify underlying causes and explain these often debilitating symptom patterns. Keeping a headache diary – noting symptoms and daily details – can also support the diagnosis.

"Why some medicines, especially painkillers, can make headaches worse isn’t fully understood. However, it’s important to be aware of this now well-established link and seek medical advice.

"Only when some patients stop taking certain medications altogether do they discover the uncomfortable truth: that their pain was being fuelled by the very drugs they depended on."