Although eating acorns in small quantities is unlikely to cause a severe medical reaction, acorns do contain tannins that cause an upset stomach when eaten in moderate to large quantities. Because these tannins also give raw acorns an unpleasant, bitter taste, acorn poisoning is rarely a concern in humans.
The tannins in acorns cause medical problems including colic, bloody diarrhea and intestinal upset in horses, cattle and dogs. Because the shells are hard to crack and the nuts are bitter, humans rarely consume acorns, and if they do, they do not consume enough of the nuts to have a poisoning effect. According to Kansas State University, acorns do become safe to eat when they are roasted; the tannin content is reduced considerably by heating.
Acorns do present a health hazard to toddlers and infants, who sometimes choke on the hard nuts when they accidentally swallow them. Additionally, acorns can present a tripping hazard when they fall off of the oak tree. Oak leaves themselves are also poisonous, as they also contain high levels of tannins. Some children and pets have suffered tannin poisoning after drinking water in which oak leaves have soaked for many days. Of this array of problems potentially caused by oak trees and their nuts, acorn poisoning is likely the least common or worrisome.