There are several reasons why green tea may cause nausea in some individuals. Here are a few possible causes:

1. Caffeine sensitivity: Green tea contains caffeine, a stimulant that can cause nausea in some people, especially if consumed in large amounts or on an empty stomach. If you are sensitive to caffeine or have a history of caffeine-related gastrointestinal issues, green tea may cause nausea or other digestive symptoms.
2. Tannins: Green tea contains tannins, a type of plant compound that can cause nausea, especially if the tea is brewed for too long or at too high of a temperature. Tannins can also interfere with the absorption of certain nutrients, which may contribute to feelings of nausea.
3. Poor quality tea: Green tea that has been stored for a long time or is of poor quality may contain mold or other contaminants that can cause nausea.
4. Other health conditions: In some cases, nausea after drinking green tea may be a symptom of an underlying health condition, such as acid reflux or an allergy to certain components of the tea.

To reduce the risk of nausea, you can try the following:

1. Limit your intake of caffeine: If you are sensitive to caffeine or have a history of caffeine-related digestive issues, try limiting your intake of green tea or choose a decaffeinated variety.
2. Avoid brewing the tea for too long: Overbrewing green tea can increase the amount of tannins present in the tea, which may contribute to feelings of nausea.
3. Choose high-quality tea: Opt for fresh, high-quality green tea to reduce the risk of consuming mold or other contaminants that may cause nausea.
4. Drink green tea in moderation: Green tea is generally considered safe when consumed in moderation, but large amounts may cause digestive symptoms such as nausea.

If you continue to experience nausea after drinking green tea or have other concerning symptoms, it is important to seek medical attention to determine the cause and receive proper treatment.