Unfortunately couldn't say (but I sure hope so) as I'm using it for Discus who should be treated monthly to keep worms etc at bay. It is highly recommended though - I checked with a few places/people before buying/dosing.
Hope it works for your Guppies too
What are you adding every month that requires a re-dose of worm medicine? New fish? Infected live food? The reason I ask is, why don't you just kill off all the worms in the tank and then not worry about it again -- worms cannot just spring up out of nowhere!
Something has to be bringing the worms back, you adding something to the tank, or you not competely killing off the infection. Worms should not be coming in with your water, if they are you need to have a very serious talk with your water company!
Is wormer plus the commercial name for praziquantel? Because praziquantel is a very good wormer, with almost no side effects at all. Hikari makes a liquid form called Prazi-Plus that is much more manageable than the powder, though both forms have the exact same active ingredients. If wormer plus is not praziquantel, you might want to try that. Just be sure to keep dosing for several days after all symptoms of the infection stop. You want to be sure to eliminate 100% of the population, and not leave a small asymptomatic population that can come back at a latter time.