Zithromax Z-Pak This was the only thing that stopped pneumonia for me when I was six and now I'm on it again for bronchitis or something. I've had severe stomach cramping. My heart has been racing since I started taking it. I'm on my fourth of five days. I'm wondering if my heart is unrelated, meaning I should go see what I can do about that, or if it makes other people all jittery/anxious with a really high pulse.