A "worrying" number of family doctors are not offering at-risk patients a preventative breast cancer drug, a charity has said.
The comments from Breast Cancer Now come as a new study found that only half of GPs were aware that the drug tamoxifen could be used to reduce the risk of breast cancer.
In 2013, the National Institute for Health and Care Excellence (Nice) recommended that women deemed to be at moderate or high risk of breast cancer should be offered chemoprevention drugs - including tamoxifen.
But a new research paper found that just over half of GPs knew that the medication could be used in this way.
The study, published in the British Journal of General Practice, found that three quarters of GPs were not aware of the Nice guidelines.
Researchers presented respondents with a series of scenarios where a healthy patient was seeking a tamoxifen prescription.
They found just 51.7 % of the GPs knew the drug can reduce breast cancer risk, while only 24.1% said they were aware of the relevant professional advice.
The authors, who assessed online responses from 928 family doctors from England, Northern Ireland and Wales, found that GPs are more comfortable in discussing, and more willing to prescribe or recommend these drugs, if supported by hospital doctors.
Commenting on the study, Baroness Delyth Morgan, chief executive at Breast Cancer Now, said: "It is extremely concerning that many women at an increased risk of breast cancer are still not being offered the choice of taking tamoxifen to reduce their risk.
"Nice's 2013 guideline recommended that it be offered to all women at medium and high risk of the disease.
"But unfortunately this is largely not being adhered to, with many GPs lacking confidence in discussing the option with patients and a worrying number not even being aware of it.
"Given the current debate on the increasing patient demand on the NHS, it is essential that we make the most of low-cost preventive measures such as tamoxifen.
"This study highlights that greater support needs to be offered to GPs in prescribing off-patent drugs in new uses.
"Ultimately, while not all women will want to take tamoxifen as it has side-effects as well as benefits, it's imperative that they are offered the choice and given all the information they need to make an informed decision."
Cancer Research UK (CRUK) called for better support for GPs.
The tamoxifen study forms part of a new CRUK report which also looks at other cancer preventing drugs.
The report highlights that 73% of GPs know that aspirin could reduce the risk of bowel cancers in people who are not at high risk.
And among GPs who had heard of Lynch Syndrome - a genetic condition which can increase a person's risk of bowel cancer, cancer of the womb and some other cancers - just under half knew that aspirin could reduce the risk of cancers linked to the syndrome.
The report makes a number of recommendations to ensure cancer preventing drugs are routinely discussed with and offered to patients who may benefit.
Report author Dr Samuel Smith, a Cancer Research UK fellow at the University of Leeds, said: "Our report helps us to understand GP attitudes towards the use of cancer preventing drugs.
"It's clear that more needs to be done to promote the evidence and guidance associated with these drugs, particularly as research reveals GPs are lacking the support to discuss effectively the risks and benefits of preventive therapy."
Professor Arnie Purushotham, Cancer Research UK's senior clinical adviser, added: "Cancer-preventing drugs have the potential to have a huge impact by reducing the risk of cancer developing in the first place.
"This report reveals that it's vital that GPs are given the right support and information so they are confident to explore the value of these drugs with those who would benefit from them, wherever they are in the UK.
"While this study focuses on reducing cancer risk, chemoprevention can also be used to reduce the risk of some cancers returning or spreading.
"It's essential that the NHS provides a clear steer to doctors to ensure all patients have equal access to treatments that could benefit them."
NHS Digital data shows there were 662,264 prescriptions dispensed for tamoxifen in 2015.
Professor Helen Stokes-Lampard, chairwoman of the Royal College of GPs, said: "The benefits of using long-term medication to lower the risk of developing cancer are becoming clearer as new research findings become available - and it's important that this informs official clinical guidelines, and that GPs and our teams are aware of them.
"But with clinical guidelines rightly being updated so frequently and given the incredibly broad spectrum of knowledge GPs need to have, it's understandable that family doctors often take cues from our specialist colleagues in hospitals - so improved communication channels between primary and secondary care would certainly be helpful.
"Ultimately, any decision to prescribe drugs to patients must be the result of a full and frank conversation between doctor and patient, taking into account the latest clinical guidelines, considering all the positives and negatives, but also based on a patient's unique circumstances and family history in order to achieve the best possible health outcome for that patient.
"Cancer is an enduring priority for the College, in partnership with Cancer Research UK, and we have developed a wide range of resources to support GPs in the timely identification of cancer, and keep up to date with the latest clinical guidelines and information in this area."