Honestly Im not sure. My pdoc said it was a strong course that would get me into the right routine. Once I was in the right routine it was much easier to keep, and I have very few issues since. It was a 21 day course and I had to get a prescription for it. I would not hesitate to use it again. I wonder though because I had never used it previously, it had more of an effect on me. Would taking melatonin PRN or regularly affect its effectiveness? Do you build up any type of tolerance to it? I wouldnt take it except in the manner I have, because it is what works for me. I also found with melatonin that I needed to go to sleep the second I felt the slightest bit sleepy. If i pushed past that then I didnt sleep, I had to go to bed AS SOON as there was any effect.