I have 2 children. I also have chronic allergies and was not able to take my normal medicine, Allerx, during my pregnancies or nursing because it was too new and there was not enough information on that it would be safe to take and not harm the baby. Zyrtec was what my OB told me would be safe. Therefore, that's what I took. I nursed both of my children until nearly a year. I work outside the home so I had to pump during the day and I froze the milk. With my first one, I was able to stop pumping by around 10 months because I had so much frozen milk saved that it didn't get used up until a month after his first birthday. My second one, I dried up a little earlier, but it was still near 10 months or after, at which point my pediatrician had us go to whole milk, which she took just fine. In my experience, Zyrtec definitely did not hinder milk production.
One secret to my success... after my kids were done with a feeding, I would hand pump in order to empty that breast. That built up how much I produced as well as the store of what they could eat/drink while I could not be with them.
Hope this helps!