How did Alexander Fleming find out that penicillin was safe for humans to use?

Alexander Fleming discovered penicillin's potential as an antibiotic in 1928 when he noticed that the mold Penicillium notatum inhibited bacterial growth. However, it wasn't until the early 1940s that researchers Howard Florey and Ernst Boris Chain conducted extensive tests on penicillin, demonstrating its safety and efficacy in humans. They performed clinical trials during World War II, which confirmed that penicillin could effectively treat bacterial infections without significant side effects, leading to its widespread use.