Hey just wanted to know any information anyone has on worming, getting worms, effects, treatment, products etc.

I ask for a few reasons.. Firstly I buy the tablets from my pet food store and it does say on the box every 3 months the tablets are around $1-$2 each. Then Ive seen in Vets they have worming tables up to $20 for a dog.. Why such a price difference? Anyway I was worming maybe once every two months but Ive found my pup has worm looking things in his poo after just one month after worming him.. Any suggestions comments would be helpful thanks