I have had the worst arm and shoulder pain after I took Zyrtec for only a few days to clear up some seasonal allergies. My upper arms hurt so bad, especially when I slept on them, which also led to some shoulder pain. My arms hurt for about 2 weeks after I stopped taking the Zyrtec. The pain started while I was taking Zyrtec. After not taking Zyrtec, the pain is finally subsiding. I will not be taking Zyrtec again; the arm pain was awful!