More than a million people in Britain suffer from "completely preventable" headaches because they over-use painkillers, experts have warned.

Patients who take drugs such as paracetamol, ibuprofen and aspirin on more than 15 days of the month could actually be making their headaches worse due to a "vicious cycle" of pain linked to their medication.

These so-called medication overuse headaches affect one in 50 of the population, according to new guidelines issued to doctors by Nice, the National Institute for Clinical Excellence.

Professor Martin Underwood, who chaired the review, said that using painkillers too regularly can cause a "vicious cycle where the pain gets worse and you take more painkillers, which makes the pain worse and makes you take yet more painkillers".

Professor Underwood added that for patients with these types of headaches "the only treatment is to stop taking painkillers" – although he admitted this was trickier to achieve than it sounds. "It's one of the more difficult consultations I've had to do as a GP as I know that the headache will get worse for a few weeks before it gets better, and it needs a very strong patient to be able to persevere with it," he said.

The guidelines, which were drawn up following an extensive review of existing research, showed that headaches generally were "underestimated, under-diagnosed and under-treated" by medical staff.

Improved knowledge of headaches among doctors would help prevent the NHS carrying out "unnecessary brain scans" according to Dr Manjit Matharu, a consultant neurologist, who helped draw up the guidelines. He estimated that in his experience as many as half of all scans are carried out simply to put worried patients' minds at rest.

"Most peoples' headaches will not be caused by brain tumours or other serious health problems, and so these should not be offered to patients solely for reassurance," he said.

There are 200 types of headache. But despite the vast majority these fitting into just four categories – migraines, cluster, tension and medication overuse – many doctors are still unable to distinguish between the symptoms, the review found.