In almost any debate about the nature of the Eucharist, you're almost guaranteed to hear John 6 brought up by those who think Jesus was speaking literally, mainly Catholics. For example, in John 6:55, Jesus says something along the lines of "my flesh is true food and my blood is true drink" (translation dependent). Catholics would argue you can't get much clearer than that.
Of course, even within John 6, there are multiple context clues that Jesus is not speaking literally. In verse 63, for example, He flat out says as much. "It is the Spirit who gives life; the flesh profits nothing. The words that I speak to you are spirit, and they are life." (Emphasis added).
But further evidence that Jesus was speaking figuratively here comes from the fact that later on, in John 16, Jesus tells the disciples a time was soon coming when He would stop using figures of speech and speak to them plainly. And their response was one of surprise. Observe: "“These things I have spoken to you in figurative language; but the time is coming when I will no longer speak to you in figurative language, but I will tell you plainly about the Father. In that day you will ask in My name, and I do not say to you that I shall pray the Father for you; for the Father Himself loves you, because you have loved Me, and have believed that I came forth from God. I came forth from the Father and have come into the world. Again, I leave the world and go to the Father.” His disciples said to Him, “See, now You are speaking plainly, and using no figure of speech! Now we are sure that You know all things, and have no need that anyone should question You. By this we believe that You came forth from God.”" (John 16:25-30).
But we shouldn't need a passage wherein Jesus promises to cease using figures of speech to tell us that Jesus used figures of speech, as He quite often used such obvious figures of speech that not even the Catholic Church insists we believe they are literal. Do we literally need to be born again? (John 3:3). Must we literally hate our families to be Christ's disciples? (Luke 14:26). Is Jesus a literal vine? (John 15:1). These are just a few obvious metaphors that not even the Catholic Church would dare say are literal, even though they're pretty firmly stated.
Why, then, is the Eucharist considered the only exception? To this very day, I can't actually figure out why this particular doctrine was invented in the first place. I can understand why children, who would believe you have stolen their nose when it is clearly your thumb, might think this is literal. But why fully grown adults would even think they were eating human flesh and drinking human blood, much less stake their eternal life on it, is far beyond me. It is far more sensible to understand John 6:55 in light of John 6:25-65, where Jesus actually explains what He means: "I am the bread of life. He who comes to Me shall never hunger, and he who believes in Me shall never thirst." (John 6:35).
When we expand our reading of scripture beyond cherry picked verses which can be twisted to make the Eucharist the literal flesh and blood of Jesus, we see that "my flesh is food indeed, and my blood is drink indeed" can be safely categorised with Jesus' many other obvious figures of speech. The Catholic doctrine of transubstantiation, which didn't really become a big thing in the Catholic Church until the Great Eucharistic Controversy in the 800s A.D., is little more than a peculiar dogma perpetuated by a false Church with an errant gospel.