i was sooo scared, but i did try and it really helps.
If you have the choice they make ones that look like teapots and others that look like squeeze bottles. the squeeze bottles are alot easier to use because you control the pressure by how hard you squeeze.
If you do it wrong your body will protect you, it will just feel like it does if you accidentally get splashed hard at the pool. but it is only a second because you stop the flow. I have a pretty severe gag reflex so if my head isn't tipped enought I do gag but only once because you stop and adjust.
I would try it in the shower first it might be easier and don't try to empty the bottle or pot just tell yourself you are going to try each nostril two times and then you give your self permission to quit. Then you'll love it and keep going.
I do think i blew my nose too hard after using it once or maybe didn't have my head tipped right and got some fluid in my ear and ended up with something that was kindof like an ear infection but not as severe. so that would be bad, but suffering through allergies is TEN times worse.
make sure your water is a comfortable temp, I put a bit on my wrist like they used ot do for baby bottles. and have a full box of tissues ready to blow it all out.
You will feel soooo much better. I promise!!