Can long term use of cetirizine cause diarrhoea

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See side effects on this link. According to the link, no diahrrea is not a common side effect. Since your doctor knows your history, and what other medications you might be on, it's best to speak to him/her about this.

Considering it is an antihistamine and, in it's actions, reduces watering eyes and sneezing, I would anticipate it might be more prone to cause constipation.