The Shulchan Aruch (name) writes: "The main mitzvah is with chazeret. And if one does not have chazeret, he should go back after the first one, one by one, according to the order in which they are changed." The Mishnah Berurah (sif mem bet) explains: "With chazeret - which is a remembrance of the slavery in Egypt, which began softly and ended harshly, and so too is the nature of chazeret, that it starts sweet and ends bitter. And the later authorities wrote, that even if it is slightly more expensive than other bitter herbs, it is still correct to honor it afterwards." And he notes: "Indeed they wrote, that in the type of chazeret (meaning lettuce) that is very common on Passover days are small worms that are not recognizable to the weak-eyed, therefore, for one who does not have special people with fear to check it properly, it is better to take tamcha (known as horseradish), even though it is third according to the order in which they are changed, for heaven forbid to transgress a negative commandment for the sake of fulfilling a rabbinic positive commandment, especially since it is possible to fulfill both with the tamcha."