How was Warfarin discovered?

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Warfarin is a commonly prescribed drug used as an anticoagulant (blood thinning agent) in modern medicine, but it wasn’t initially produced for this purpose. It works to reduce clotting of the blood by blocking the enzyme ‘vitamin k epoxide reductase’. This basically reduces the amount of active vitamin k which in turn reduces the ability of several clotting factors in the blood to perform their function.

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Warfarin was first used in 1948, but not for medical purposes – it was used as a rat poison. It was 6 years later that it was first approved for use in humans in the USA. But the initial discovery dates back to the 1920’s in northern America and Canada. Cattle were afflicted with an illness which was causing internal bleeding and subsequently death in large numbers. It was found that it was related to the cows being fed with damp hay (sweet clover) which would previously never have been used – it was simply due to financial hardships at the time. Despite this new knowledge many farmers continued to use the mouldy hay as food for their livestock and the disease remained prevalent. Eventually a biochemist called Karl Link began the work of identifying exactly what the anti-clotting agent was and by 1940; Link and his team had established what the naturally occurring substance was. Coumarin was oxidised in the damp hay to produce a substance called dicoumarol.

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In the mid 1940’s the possibility of dicoumarol as a poison against rodents was investigated by Karl Link but he discovered it to be too slow acting, so it was modified many times until a similar substance was produced and named Warfarin.

Following its success as a rat poison, it was soon discovered to be useful in humans as a clinical anticoagulant. Other drugs such as heparin were already available, but Warfarin offered some different advantages, and when it was found that its effects could be reversed using vitamin k it was deemed safe for use in humans.

This method of drug discovery is a far cry form the modern clinical trials employed today. There are much more rigorous testing procedures in place to ensure the safety of medicines prior to their use in the wider population, starting with adaptive phase 1 studies and only progressing to phases 2 and 3 and licensing if they have proven their benefits outweigh and notable risks. Discover more about this at

These days people who have had a previous history of blood clots (such as deep vein thrombosis or pulmonary embolism), or who have a known increased risk for one developing, are often prescribed Warfarin. The NHS lists some of the key risk factors for clots as; atrial fibrillation, some blood clotting disorders, people with replacement heart valves and in some people following surgery and a higher chance post-operatively of forming a clot.

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