Wintergreen is almost pure methyl salicylate, at 96 to 99% of the oil. In the body that turns into salicylic acid, the active part of aspirin, which is why it eases pain and swelling.^[1] It also means the risks are aspirin's risks, amplified. It passes through skin easily and can reach blood levels high enough to thin the blood.^[2] Swallowing it or over-applying can cause salicylate poisoning, and painkillers taken at the same time make that worse.^[2]