You can start from 2 weeks of age if you use a mild, liquid wormer such as Panacur suspension. If you want to use a tablet, I would say from about 3 weeks of age.Then it is best to repeat the worm dose every 2 to 4 weeks until about 12 weeks of age. Once a month from 12 weeks to 6 months is good. After that, twice a year is OK for most cats.
It varies enormously just how many worms any individual kitten/cat will be prone to get. Some kittens can almost die with very heavy worm burdens, others will get very few even if they are not wormed at all. As long as you use a safe wormer, it is best to follow the guide above.(Kittens only really get round worms, so you do not need to bother about tapeworms.)
Sunday 20th January 2013