What is a safe level of probiotic that should be taken. i was currently taking 13 billion 1 time a day it's causing stomach issues ?

Try for a: refrigerated probiotic, or off the shelf if traveling. Don't be overly concerned w/the numbers, i.e., how many billions in each dose. Look more for a variety of ingredients, i.e., bacteria/yeast in each bottle. PRE-biotics prompt the body to make PRO-biotics and can be good also. Then you would look for l-glutamine which does not have to be refrigerated. Peace and good health.