Green tea has been proven highly beneficial in the treatment of acne, comparable to a 4% benzoyl peroxide treatment without the same, negative side effects.
There are three ways in which green tea can help to prevent and treat acne.
1. Green Tea Reduces Sebum Production
Acne is often caused by over-production of sebum formed by the sebaceous glands. One of the ways in which green tea helps to treat acne is by reducing sebum production.
The EGCG component of green tea helps to counteract this by interfering with the IGF-I signals which encourage sebum production.
According to a study done by Myung Im, et al. it was “demonstrated that EGCG suppresses IGF-I induced sebocyte proliferation, lipogenesis, and cytokine expression. Taken together with our supplementary data, this preliminary evidence suggests that EGCG could be an interesting candidate for the treatment of IGF-I aggravated acne.” 
2. Green Tea Has an Anti-Inflammatory Effect
According to Pubmed “The inflammatory stage of acne vulgaris is usually of greatest concern to the patient. A number of morphologically different inflammatory lesions may form that can be painful and unsightly.”
The EGCG in green tea is well known for its antioxidant properties and so may be beneficial in the treatment of acne to help reduce inflammation and stop pro-inflammatory responses.
3. Anti-Bacterial Effects of Green Tea
Acne may be caused by the P. Acnes bacteria and so studies have been done to show whether the antibacterial and antimicrobial effects of green tea may be beneficial against this particular form of bacteria as well.
An article in the Journal of Investigative Dermatology by Jo Young Yoon, et al. investigating the effect of EGCG on acne states that:
“EGCG significantly decreases mean colony-forming units in a dose-dependent manner, suggesting that EGCG suppresses the growth of P. Acnes. These results suggest that EGCG could target the abnormal colonization of P. Acnes in the development of acne.” 
How Green Tea Can Be Used for Acne
Green tea can be taken in a number of forms to aid in the treatment of acne. First, a number of topical applications have been created in which green tea extracts can be used externally on the skin to treat acne. Including green tea in your diet is another solution and may help treat the acne from the inside out.
Green tea has been seen to have three primary roles in the treatment of acne, namely, decreasing sebum production, acting as an anti-inflammatory and preventing the abnormal colonization of P. Acnes bacteria which causes acne. Most of these effects are due to the presence of EGCG, a strong antioxidant present in green tea.