I have been doing it (drenching) a lot of sheep for years and have never had even one problem!!
As long as you don't lift their mouth up to the sky, basiclly keep it a little higher than how they usually hold it, should be fine!!!
The only pellets I've seen are labeled for goats. Would be nice to have them for sheep but at least with drenching, you can make sure they are actually getting the right dosage and you don't have to worry about whether they'll eat it or not.
I was nervous about "drenching" maybe because of the name and did use the safeguard pellets a couple times. Finally decided to drench and so much easier than trying to make sure everyone gets the right dose. I just squirt it in their mouth and it's done. I use ivermectin.
I was extremely nervous at first (probably because I read too many "warnings"),
but after doing it the first time, I felt quite confident.
I give my sheep a regular (every month or so) drench with garlic/molasses/probiotics,
so they are very used to the drench tool. I use the molasses because they LOVE the taste
and actually look forward to sucking on the drench tool (a few of my sheep actually come back for more).
So if I do ever have to use the drench tool for some type of medication, it won't be too difficult.
I stick the drench tool in at the corner of their mouth past the back of their tongue and squeeze gently.
If you have a large dose, allow them time to swallow as you squeeze the fluid out.
There are also some good how-to videos online like this one: HOW TO DRENCH