Primal Defense and Threelac are both probiotics. They are among the most popular used by people with digestive problems. I buy my Primal Defense at my local health store, but you might be able to get a better price if you buy it online. Primal Defense has many strains of Homeostatic Soil Organisms than you will find from the more inexpensive probiotics that are available--like the ones that need to be refridgerated.
Primal Defense doesn't attack candida like Threelac does, although it is still good, but it doesn't have the side effects either. On the good side, if you do liver flushes, you will get benefits you would have gotten from Threelac(and more), only on a more permanent basis. With the flushes, you are actually cleansed and Threelac doesn't make you better that way. I'm not sure "what" it's doing to make you feel better since there are side effects...your guess is as good as mine. I thought I discovered an amazing hidden treasure when I first started 3lac and suddenly felt SO MUCH BETTER. What a shame.