Don’t miss a day once you start taking it. It’s not an instant relief med, but has to build in your system for about 24 hours. In my experience, for example, missing Tuesday meant a headache on Wednesday.
Zyrtec and Allegra, I believe, provide same day relief. But I still prefer Claritin myself, for various reasons.
The bigger concern typically comes from using non sterilized/distilled water. Overall, and with proper use and cleaning, it’s an effective and safe method of sinus cleansing.