There are so many different kinds of vitamins out there and between price and "are you getting everything you need" I went through a few brands. When I began taking One A Day Womens, I could tell there was a difference in the potency. I did feel more energetic and tried hard to make sure I used them in a balanced diet. It was about two weeks after taking them, I started to notice that I was very edgey,quickly irritated, and having mood swings--very similar to PMS, but I was not even close to that point. I then forgot [as we all do] to take my vitamins and the edginess went away. But then, when I remembered to take the vitamins, the irritation and short temper came back. This time I was paying attention and noticed that I just didn't "feel right" in my body. Upon some research I discovered that each vitamin company uses different fillers in their vitamins and that you could have a physical reaction to them. I stopped taking those vitamins and switched to another brand and have never had the same problem since.