Patients, physicians, pharmacists and other healthcare providers need to know about the potential harm these herbs can have.

Millions of people are thought to use over the counter herbal remedies but doctors have warned that just because they are 'natural' does not mean they are safe.

They added that these substances can interfere with the way conventional medicines work, making them more potent or ineffective.

Researchers writing in the Journal of the American College of Cardiology said patients should inform their doctor if they are taking any herbal preparations.

Experts from the prestigious May Clinic in Arizona, America, said that St John's Wort, typically used for mild depression and sleep problems can reduce the effect of drugs to treat heart rhythm problems and high blood pressure. It may also increase blood cholesterol levels which may contribute to heart problems.

Ginkgo biloba, taken by some to boost their immune system or 'sharpen' their attention and increase energy levels can increase the risk of bleeding in patients taking blood thinners warfarin and aspirin.

Garlic can also have the same effect when taken in large doses.

Patients taking drugs such as warfarin are already wanred to avoid a list of foods and drinks, including grapefruit juice, as it interferes with the blood thinning action of the drug.

Dr Arshad Jahangir, Professor of Medicine and Consultant Cardiologist, at the Mayo Clinic, said: "Many people have a false sense of security about these herbal products because they are seen as 'natural'.

"But 'natural' doesn't always mean they are safe. Every compound we consume has some effect on the body, which is, in essence, why people are taking these products to begin with.

"We can see the effect of some of these herb-drug interactions some of which can be life-threatening—on tests for blood clotting, liver enzymes and, with some medications, on electrocardiogram."

He said a major concern is that patients often do not tell their doctor they are taking herbal remedies as they do not think of them as medicines.

Dr Jahangir said: "These herbs have been used for centuries well before today's cardiovascular medications—and while they may have beneficial effects these need to be studied scientifically to better define their usefulness and, more importantly, identify their potential for harm when taken with medications that have proven benefit for patients with cardiovascular diseases.

"Patients, physicians, pharmacists and other healthcare providers need to know about the potential harm these herbs can have."

Proton pump inhibitors (PPIs) may reduce the benefits of aspirin in patients with coronary artery disease (CAD)

Proton pump inhibitors (PPIs) may reduce the benefits of aspirin in patients with coronary artery disease (CAD), researchers from Denmark report.

Previous research has suggested that concomitant PPI use might be responsible for the reduced aspirin efficacy seen in some patients.

“Aspirin is the mainstay of secondary antithrombotic treatment; accordingly, low-dose aspirin lowers the risk of vascular events by 32% in high-risk patients,” explain Anne-Mette Hvas (Aarhus University Hospital, Brendstrupgaardsvej) and colleagues in the journal Heart.

“However, platelet response to aspirin is variable and in some patients platelet aggregation is inhibited less than expected and these patients might be at an increased risk of cardiovascular events.”

PPIs exert their antacid effect by inhibiting an ATPase associated with gastric parietal cells and raising intragastric pH, which can greatly reduce the lipophilicity of aspirin and, therefore, potentially its efficacy.

To better understand the effects of PPIs on aspirin function, the investigators studied 418 patients with stable CAD, all of whom were taking aspirin 75 mg/day and 54 of whom were also taking PPIs.

Analysis revealed that platelet aggregation was significantly greater in patients taking aspirin as well as a PPI (180 units/min) than in those taking aspirin only (152 units/min).

Platelet activation, as assessed by levels of soluble serum P-selectin, was also significantly increased in those taking both aspirin and a PPI (88.5 ng/ml) than those taking aspirin only (75.4 ng/ml).

Importantly, these differences were not explained by differences in age, sex, body mass index, blood pressure, family history of ischemic heart disease, smoking, or diabetes, all of which factors were well balanced between patient groups.

Although the authors did not demonstrate a causal link between PPI use and reduced aspirin function, “these findings may nevertheless affect the clinical practice of antithrombotic treatment,” conclude the researchers.

“In view of the widespread use of PPIs, a randomised double-blind crossover study is needed to explore further the inhibitory effect of PPIs on aspirin,” the investigators recommend.