Matcha green tea is safe to drink. Yet, although rare, there are some possible side effects to watch out for, including upset stomach, suppressed appetite, and caffeine-sensitivity:
Caffeine - Like coffee and other tea, matcha also contains caffeine. The average serving of matcha contains about 35mg of caffeine, whereas coffee is about 120-150mg (they are chemically different too). Still, be sure not to overdo it.
If you have a reason to be careful about caffeine consumption, it’s suggested to consult with your physician first.
Upset Stomach - The natural antioxidants in tea, one group known as tannins, may cause occasional upset stomach for some. Premium quality matcha tea is higher in tannin content (more details here) because it’s grown well. While good for you, may cause irritation, usually to newcomers.
If you experience this side-effect, be sure you’re sourcing a high-quality matcha. And consider enjoying your matcha following breakfast or lunch.
Appetite - Since matcha is a calming, yet stimulating beverage, it’s possible that it will delay your sense of hunger. This mostly has to do with the caffeine, but it may also be due to the nutritional value of matcha. It can help keep you full with vitamins, trace minerals, and amino-acids.
Antioxidants like EGCG in matcha are also studied to elevate fat-burning, which may be responsible for suppressed appetite.