My daughter is two years old and has on and off diarrhea for the past year. She sometimes will go two to three weeks before it clears up. We have tested for everything and all shows up negative. Any advice?

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What does her pediatrican say?

Toddler diarrhea. What is this??

Two weeks of diarrhea is way too long. I would bring my child to a doctor after 2 days.

Sometimes it is a food item that can cause digestion problems. Dairy is a big culprit... if you haven't already, you may want to see if cutting back on her dairy intake makes any difference. Dairy can stay in your system for awhille but if it is a possible cause, you should see some change after several days.
You can make sure she is getting the calcium and vitamin D that she needs in other ways other than dairy products if that does turn out to be an issue for her.