According to Catholic doctrine, when the bread and wine consumed during the Eucharist are consecrated by a priest, they literally become the flesh and blood of Jesus Christ. This is called Transubstantiation. This doctrine is a fundamental difference between Catholicism and "Protestantism", and is one of the main battlegrounds. If Transubstantiation is true, Catholicism is true, as not only do they have the correct view of salvation, but also the authority to go along with it. If Transubstantiation is false, Catholicism is false, as it teaches a false gospel, and therefore its leaders are anathema (Galatians 1:8).
One argument Catholics use to substantiate their doctrine is Eucharistic miracles. Ordinarily, even the most die hard Catholic will admit that the priest can consecrate the bread and wine all he likes, it remains bread and wine. It looks like bread and wine, it acts like bread and wine, it tastes like bread and wine, and its final destination is the toilet, much like bread and wine. But according to the Catholic Church, there have been exceptions throughout history. Times when the host, rather than remaining bread and wine, visibly transform. These Eucharistic miracles, such as the ones portrayed in Italian documentary "Segni" (signs) are supposed to prove the Eucharist.
The first problem with this argument is that Eucharistic miracles are dubious, to say the least. Indeed, many of them are outright refuted. Several of them turn out to be fungus rather than blood. That's when research actually happens, but quite often, it is prevented from doing so. You will struggle to find any properly documented examples of scientific studies into Eucharistic miracles.
One scientific study I would like to see done is not in the form of biological investigation, but psychological. Take a Eucharistic miracle, place it in a room full of high ranking Catholics (even the Pope, if he is available) and point them to it. So, you believe this is the actual flesh and blood of Jesus? You believe it's what you've been consuming every Sunday since you converted? You believe your religion actually requires you eat it to get saved? Put your mouth where your money is: Eat it.
I'm willing to bet that very few of them, if any, would volunteer. They might not know what it actually is, but they know what it isn't: The resurrected, glorified, incorruptible body of Jesus Christ. Some of these "miracles" are hundreds of years old. Who knows what would happen to the body of a man so foolish as to stick them in his mouth?
But if it really is the flesh and blood of Jesus, it should be incorruptible. The Bible says that Jesus' body will not see decay (e.g. Acts 2:31). It is absurd enough to think Jesus' actual body could be eaten up and pooped out, since His body is no longer perishable (1 Corinthians 15:42), and He is forever alive in Heaven (Romans 6:9). But if you think His body is partially and permanently here on Earth in the form of Eucharistic miracles, you must believe it is immune to the natural decay of earthly objects. So you have two choices. You can eat it and get very sick, or you can refuse and admit you know whatever it is, it isn't Christ's physical body or blood.
But the strength of the Christian faith does not come from science. Science may be able to investigate Eucharistic miracles, but it need not. We can actually afford to let every single Eucharistic miracle, no matter how spurious, be considered genuinely miraculous and still reject the conclusions drawn from them. Why? Simply because Satan is perfectly capable of performing miracles if God allows him to.
In the Catholic NABRE, Deuteronomy 13:1-4 reads "Every word that I command you, you shall be careful to observe, neither adding to it nor subtracting from it. If there arises in your midst a prophet or a dreamer who promises you a sign or wonder, saying, “Let us go after other gods,” whom you have not known, “and let us serve them,” and the sign or wonder foretold to you comes to pass, do not listen to the words of that prophet or that dreamer; for the Lord, your God, is testing you to know whether you really love the Lord, your God, with all your heart and soul." From this, we see that even false prophets can perform some very convincing miracles, but if they preach falsehood, they must not be heeded. Keep that in mind when we read Matthew 24:23-27:
"If anyone says to you then, ‘Look, here is the Messiah!’ or, ‘There he is!’ do not believe it. False messiahs and false prophets will arise, and they will perform signs and wonders so great as to deceive, if that were possible, even the elect. Behold, I have told it to you beforehand. So if they say to you, ‘He is in the desert,’ do not go out there; if they say, ‘He is in the inner rooms,’ do not believe it. For just as lightning comes from the east and is seen as far as the west, so will the coming of the Son of Man be."
This passage has direct implications for the Eucharist as a whole, not only for the alleged miracles. Here, we're not talking about people leading us to false gods, but to the Messiah (i.e. Jesus). But these are not people to whom we should listen. After all, Satan's ministers portray themselves as ministers of righteousness (2 Corinthians 11:14-15). But notice, they can perform such convincing miracles that even the elect might be deceived.
But what is of particular note is what these false prophets are telling us to believe. They want us to believe "here is the Messiah". But when the Son of Man comes, it will be "as lightning". There will be no mistaking it when Christ comes. But according to Acts 3:21, that won't be until the restoration of all things.
My Catholic friends, He isn't here! Jesus' physical body is not in Eucharistic miracles, nor is it in the bread and wine you drink on Sunday. As if some priest had the authority to summon Jesus. "Here boy, c'mon, come here Jesus, whistle whistle". No, Jesus remains in Heaven. If the Catholic Church tells you "here He is, in this bread", scripture says "don't listen to them". If they bring signs and wonders to "prove" it, scripture says many false prophets will arise. Eucharistic miracles are no kind of argument for the false doctrine of Transubstantiation, as the word of the Living God testifies against it, and against them.