One who realizes after eating food that he has eaten a harām meal, in case there was no likelihood of the food being harām and there were signs indicating that it was halal, for example, he had received the food from a Muslim, such a person has not committed a sin and he is not considered to be sinful. However, if the food had been doubtful such as when he received it from a non-Muslim individual, in which case, it is his duty to conduct an investigation to make sure that the food is halal. It is not permissible to eat food unless one is certain that it is halal. In case there is no sign indicating that the food is halal or harām, the general principle applicable to meat is that it is harām and impure. Therefore, one who wants to eat it must make sure that it is halal. Such a principle is not applicable to other foodstuffs. However, if a person who does not know whether a certain food product is harām or halal and he is guilty of not conducting a research (i.e. he could make a research but he did not do so due to laziness), he has committed a sin. But if a person is ignorant and he is not guilty in his ignorance (that is, it has not been within his means and ability to investigate), he has not committed a sin. In fact, in places where a person does not know whether or not a certain object has been made impure) if a person is guilty of his ignorance also, he has not committed a harām act.
Although such acts may take place erroneously, based on many narratives passed on to us from the Ahlulbayt (a.s.), they leave their negative impacts on man's life causing him at times to be deprived of success. Therefore, one must be careful to eat halal food and with this care and concern he should speed up his move in the path prescribed by God.
If a person knows that the meat or food is harām, i.e. he knows the food and also the rules applicable to it, yet he eats it, he has committed a sin. His action is defined and considered as an act of "defiance against God's law", and there is no doubt that it will entail divine punishment. Obviously, this sin leaves its negative impacts on the man's life.
Imam Sadeq (a.s.) says about the destructive impacts of sins, "The effect of this sin is that it deprives [the sinner] of worshipping. Sometimes, a man commits a sin at day time as a result of which he becomes deprived of staying awake and offering Night Prayers except for when he repents after his iniquity and reforms (himself).”
If a person eats such food without knowing that it is harām, there are two suppositions that might be applicable to it:
1. He did not know the thing which he has eaten; that is to say, he knew that eating pork was harām but he did not know that the food which he ate contained pork. He came to know about it after he ate it. There can be three probabilities here:
a) The food had not been doubtful, so he did not think that it was harām; rather there had been signs indicating that the food was been halal, for example, he had received the food from a Muslim, such a person has not committed a sin and he is not considered to be sinful, if later on it becomes known that the food had been harām.
b) The food has been doubtful. For example, he has received it from a non-Muslim individual, in which case, it is his duty to conduct an investigation to make sure that food is halal. It is not permissible to eat the food unless one is certain that it is halal. Hence, if a person eats the food without investigation and later on it becomes known that the food had been harām, he has committed a sin.
c) There is no sign indicating that the food is halal or harām. Since conducting investigation is not necessary in such cases, if it becomes known, after eating the food, that it had been of the harām type, he has not committed a harām act. In fact, the general principle applicable to meat is that it is harām and impure. Therefore, one who wants to eat meat must make sure that it is halal. Therefore, if a person eats meat without investigation and later it becomes known that it had been harām, he has committed a sin.
2. He did not rule (order) about the food but he knew what the food was. In other words, he did not know that eating pork was harām but he knew that the existing food contained pork, in which case, if he could have conducted a research and found the truth but he did not do so, he is considered to be sinful and his act of eating the food had been harām, but if he had not been able to investigate about the food or he did not have the means and ability to learn the rules, in this case, he is not considered to be guilty and sinful and his act of eating the food will not entail punishment for him. Because as per the Prophet's saying (hadith), he is relieved of obligations and he will not be punished. He is then obligated only to purify his hand, mouth, dishes and utensils which he used to eat the harām food.
Certainly, eating harām food will leave its natural and implicit effects on the sinner's soul and body. The scholars of ethics have spoken at length about the explicit and implicit impacts of eating such types of food and they are of the view that although one is not punished for eating the food, he will not escape its effects. That is, he will be deprived of some chances of success and it will affect his soul in an unignorable way. That is why, it has been greatly emphasized in the Quran that one should be careful about the food he eats.
Someone brought the Holy Prophet (pbuh) a bowl of milk to drink but he did not drink it until he made sure that it was halal. "The prophets before me were ordered not to eat anything but halal food and not to do a task but a befitting one."
The infallible imams have warned their followers about the impacts of food on children.
Therefore, it is necessary for a man to make sure that the food he eats is halal because the food will leave an effect on his spirituality and, as a result, he will be hindered from moving on in the path of guidance or the path with which God is pleased. As one's moral behaviors, actions and the actions of a community have an effect on the events of the world, in the same way the events of the world have an effect on one's morality, behaviors and indisposition.
 - Javadi Amoli, Abdullah, Stages of Morality in the Quran, pg.153.
 - Al-Maedah: 39
 - The Prophet said: Truly Allah has for my sake pardoned the mistakes and forgetfulness of my community, and for what they have done under force or duress." Behar al-Anwar, vol.5.
 - Al-Durr al-Manthur, vol.6, pg.102; Mizan al-Hikmah, vo.3, pg.128, Muhammadi
 - Behar al-Anwar, vol.100, pg.323, vol.101, pg.96, Imam Ali (a.s.) said: Keep away your children from the milk of lewd and mad women for it leaves an effect [on them].
 - Javadi Amoli, Abdullah, Mabadi Akhlaq dar Quran [Elements of Ethics in the Quran], pg.108