Phase 4 (optional): 100,000 target disease pts (maybe some bad side effects in phase 3, not known long enuf in phase 3, most likely the public will be unlikely to use it ...)

Ex: Tacrolimus is an immunosuppressant drug that is used in organ transplant. It is then approved by FDA to treat atopic dermatitis. Then, they realize it can cause skin cancer. Then, consumers don't want to use this drug that much anymore. However, it's still a common drug for transplant pt (these pt are very sick they need organ and drugs to help their body). Of course, use of this drug in children is much more limited.