There was once a myth that overnight tea should not be consumed, as it could cause cancer. The reasoning was that overnight tea contains secondary amines, which can transform into the carcinogen nitrosamine. However, this claim lacks scientific basis because secondary amines are widely present in various foods, particularly in cured products. For example, bread typically contains 2mg/kg of secondary amines. If bread is a dietary staple, daily intake of secondary amines from bread alone would be 1-1.5mg.

The amount of secondary amines ingested from tea leaves through drinking tea is only 40% of that from staple bread, which is negligible. Furthermore, secondary amines themselves are not carcinogenic; they require the presence of nitrates to form nitrosamines, and only when reaching a certain threshold do they become carcinogenic.

Drinking tea provides tea polyphenols and vitamin C, which effectively block the synthesis of nitrosamines in the human body, acting as natural inhibitors. Therefore, drinking tea or overnight tea does not cause cancer.

However, from a nutritional and hygienic perspective, tea exposed to air for too long can easily breed spoilage microorganisms, causing the tea to sour and spoil. Additionally, nutrients like tea polyphenols and vitamin C in tea are prone to oxidation and reduction over time. Thus, while overnight tea is not harmful, it is generally better to drink tea freshly brewed.

Canned tea beverages on the market are safe to drink, as they are produced with added antioxidants and undergo strict sterilization, similar to other cold beverages.