To answer your question, I really don't think you can OD on probiotics. There are 10 times more bacteria in your body then there are cells...or about 10 trillion. So if you eat a few billion more than you need, I'm sure your gut won't mind. You could be wasting your money on something you don't need, however.so do most people just use it when they feel a sickness coming on or for more of a preparing or what?
Your intestines are supposed to be populated by billions of microorganism including many strains of yeasts and bacteria. When a person takes a round or two of broad spectrum antibiotics, this colonization of the intestines is severely depleted or completely wiped out. The gut is then left empty for the stronger and more aggressive microbes to come back in and repopulate at a much faster rate. The most dominant microbe in most cases is Candida Albicans. This particular microorganism when left unchecked by the other microbes which should be present, will take over and mutate. It grow roots or rhizomes that grow through the lining of the intestinal walls and cause a condition known as leaky gut. Candida overgrowth and leaky gut can lead to a multitude of other problems. For more info on these, read the links below. http://welltellme.com/discuss/index.php/topic,1400.0.htmlhttp://welltellme.com/discuss/index.php/topic,603.msg5890.html#msg5890
This is where probiotics come in. These are supplements (pills, yogurt, kefir, sauerkraut, kombucha and other lacto-fermented foods) which repopulate the "good guys" to keep the "bad guys" from getting out of control. Some microbes eat other microbes, but mostly they just compete for resources, much like grass fights with weeds to take over the yard. If you know anything about lawncare, the better quality your grass is, the less likely you are to have the weeds take over.
Taking probiotics can also help alleviate intestinal sickness, but uping your "good guys" to fight off such germs as e-coli, staph, strep, and salmonella. If you keep your gut in order, then most of these "germs" wouldn't make it to the stage where the body actually gets sick, but if you've damaged your natural intestinal flora and thereby lowered your immunity, probiotics are necessary to get everything back in working order.
Hope this helps.