Here is a copy/paste of what I found:
Could eating an acorn make my dog sick?
- Bob B.Acorns (Quercus spp.) contain a toxic principle called Gallotannin. In cows and horses who repeatedly ingest significant amounts, we have seen potentially severe gastrointestinal irritation, depression and kidney damage.
Dogs, however, generally do not forage on acorns as livestock do—and even if they do ingest several acorns, it is usually an acute (single) exposure, not a chronic situation. In these cases, we typically only see mild to moderate gastrointestinal upset, which can include vomiting, diarrhea and abdominal discomfort. However, there is also the potential for mechanical irritation (from the sharp fragmented pieces of acorn), and possible obstruction, should a large amount of acorn material become lodged in the GI tract.