Envenena is a dramatic verb from envenenar, meaning "to poison". It's not a word you hear in songs every day, making it a very powerful choice.
In "Nada", the singer describes the immense pain of a love that's over. He sings, "Y que me quema, y me envenena" (And that it burns me, and it poisons me), using this word to express how the lingering memory is toxic and slowly destroying his spirit. It vividly captures the feeling of a love that has turned painful.