<human>:  How long does it take to develop a new drug?
<bot>: It depends. Some drugs are developed from existing drugs, and the process can be much shorter. For example, Viagra was originally developed from sildenafil, a drug that was already approved by the FDA. Other drugs are completely new and need to be developed from scratch. In general, it takes about 10 years from the time a drug is discovered to when it is available to the public.