Understanding the argument - We need to strengthen the conclusion that "the new drug is clearly preferable" even in light of its drawbacks, which is mild nausea vis-a-vis the serious side effects of the "most commonly prescribed" such as seizures.
A DmitryFarber beautifully explained, option E, in a way, also takes away that mild side effect. If you see the argument in light of option E, then on one side, we have the "most commonly prescribed" drug leading to seizures, and on the other side, we have a new drug with practically no side effect (with the nonprescription medication to prevent nausea). Isn't it a no-brainer to choose the new drug in that case? That's why the author needs option E. For option B, it may also be possible that "the most prescribed drug" also leads to, say, vision loss, but how does it matter as we can also create such "n" possibilities for the new drug as well? It's good to stay within the scope of the argument: whether a new drug is better; why? Understanding the core of this argument is also very important from the business application perspective. Suppose you ask your team or your boss about A (which is what the conclusion asks us to strengthen), and the other side starts telling the drawbacks of B. Does it sound familiar?

So let's stick to the problem statement we are trying to solve, which is strengthening why the new drug is preferable.
A) people who experience nausea are prone to discontinue use of the new drug prematurely - out of scope.
B) It is possible that the drug now most commonly prescribed has side effects that have not yet been attributed to it - "other" side effects we don't care about for this argument. Out of scope.
C) other drugs for the disease have typically been prescribed only for patients allergic to the most commonly prescribed drug - "other" drugs out of scope. Please don't take it as a rule; wherever we see "other," we mark it wrong. Be cautious, as the GMAT is good at creating problems that can deceive us.

. Understand the meaning.
D) people who have received effective treatment for disease do not generally contract the disease again - treatment by which drug is "the most commonly prescribed" or the new drug? Distortion.
E) there is a nonprescription medication that, when taken with the new drug, prevents the onset of nausea - ok.