Does coffee or tea cause cancer?

In 1991, the International Agency for Research on Cancer classed coffee as "possibly carcinogenic" (Group 2B), because a few studies had seen a link with bladder cancer ^[3]. It later became clear that many of those studies had not properly accounted for smoking, which is a major cause of bladder cancer and often goes together with coffee drinking ^[3].

In 2016, 23 scientists from different countries re-examined more than 1,000 studies in humans and animals and moved coffee to Group 3 ("not classifiable") ^{[1, 2]}. This group means the evidence is not enough to conclude whether coffee causes cancer; the agency stresses that it does not mean "proven safe" either ^[3]. In many studies, coffee was not linked to pancreatic, breast or prostate cancer, and it was linked to a lower risk of liver and womb cancer; for more than 20 other cancers the evidence was not enough ^{[2, 3]}.

The type of coffee (instant, filter, espresso) or the way it is made has not shown a consistent difference in cancer risk in the studies so far ^[3].

Tea itself has not been classed as carcinogenic (Group 3) ^[3]. But in 2016, drinking any "very hot" beverage, meaning above 65 °C, was classed as "probably carcinogenic" (Group 2A) ^{[1, 2]}. In studies in China, Iran, Turkey and South America, where tea or maté (a herbal infusion) is traditionally drunk very hot (about 70 °C), the hotter the drink, the higher the risk of oesophageal cancer ^[2]. In experiments on mice, very hot water, and not maté itself, also increased the growth of oesophageal tumours ^[3].

According to the agency's director, it appears to be the temperature of the drink, rather than the drink itself, that is responsible ^[2]. The usual drinking temperature of tea and coffee in most parts of the world is below 65 °C ^[3].

Golestan province in north-eastern Iran has one of the highest rates of oesophageal cancer in the world (Oesophageal cancer in Iran and Golestan province). Almost everyone there drinks black tea, on average more than a litre a day ^[4].

• A comparison of 300 patients with 571 healthy neighbours: compared with people who drank their tea lukewarm or warm, drinking very hot tea was linked to a risk about 8 times higher. Drinking tea less than 2 minutes after pouring, compared with 4 minutes or more, was linked to a risk about 5 times higher ^[4].
• More than 50,000 people followed for about 10 years: this time the researchers measured the tea temperature themselves. People who drank 700 ml or more of tea a day at 60 °C or hotter developed oesophageal cancer almost twice as often ^[5].
• In a large study in China of about half a million people, very hot tea was linked to a higher risk especially together with smoking or alcohol; people who drank very hot tea and also drank alcohol every day developed oesophageal cancer about 5 times as often ^[6].

In studies that followed millions of people for years, those who drank more coffee developed liver cancer less often; with every two extra cups a day, the risk was about a third lower ^[8]. A similar link has been seen for womb (endometrial) cancer ^[9]. A large review of all the evidence concluded that coffee at usual amounts seems generally safe and is more likely to benefit health than harm it ^[7].

But these studies only looked at people's habits and do not prove that coffee itself prevents cancer; the researchers rated their confidence in the liver cancer evidence as low ^[8]. So do not start drinking coffee to prevent cancer; but if you enjoy coffee, these studies have not shown a reason to give it up because of cancer ^{[3, 7]}.

Acrylamide is a substance that forms when some plant foods, such as potatoes, are cooked, fried or roasted at high temperatures; it is found in chips (French fries) and crisps, bread, biscuits, breakfast cereals and coffee ^[10]. In mice, large amounts of it have raised the risk of several types of cancer ^[10]. But large studies in people have not found consistent evidence that acrylamide in food increases the risk of any cancer ^[10]. Coffee itself was not classed as carcinogenic in the 2016 review ^{[1, 3]}.

To reduce acrylamide in food: do not fry or toast potatoes and bread until very crisp or brown, blanch potatoes in boiling water before frying, and do not store potatoes in the fridge ^[10].