If you do a search on this site under worming or worms or wormer there will be lots of posts with various products mentioned so that you can see which you prefer. I've used Ivomec Eprinex - lots of info about that will come up as well..
JJ
Gross, I know!!! I sure hope I don't come back as a roundworm.
You shouldn't need to worm monthly. Annual may do the trick (depending upon lots of factors), with perhaps pumpkin seeds in other months......
JJ