GetMouseButtonDown not seeming to work but GetMouseButton does?

So i want to detect when the mouse button is down. So i use if(Input.GetMouseButtonDown(0)) m_bTxtBubbleLarge = true; However this is not working at all. There is nothing that registers. If i use if(Input.GetMouseButton(0)) m_bTxtBubbleLarge = true; this will work but obviously works with both down and up... i just want it to work when its down. Any ideas?