While the leaves that grow on pecan trees are considered to be poisonous if ingested, the nuts themselves are not, at least to humans. There are certain animals such as dogs that should not eat these nuts because of the fact that they tend to cause gastrointestinal problems such as diarrhea, upset stomach, gas, and bloating.
The nuts themselves are okay for people to eat, except for those with a pecan allergy. Those who are allergic to pecans and other types of nuts should avoid eating them altogether because of the fact that they can experience a wide range of unpleasant symptoms, some of which are listed above.