Vet Meds vs Rx Meds

I am noticing a huge difference between the vet medications vs the prescriptions meds in terms of killing and control. When I was on the the expensive prescriptions, I felt like the worms were dying. Although the die-offs were strong, I felt like it was working. That's why I took a few weeks off the meds thinking I had defeated the bastards but they would come back (I didn't know I had to consistent dose for months on end but anyway). I switched to the vet meds since May/June and am feeling the total opposite. I only get relief when I introduce things (fenbendazole or source of life for example) for the first time for the first day or so then it's back to the pain. Thinking about switching back to the Rx even though it is 10 to 20fold more expensive.

Any thoughts? Has anyone consistently stayed on Rx meds battling disseminated roundworms?