GMAT Critical Reasoning (CR) | Re: A new drug, taken twice daily for one month, is an effective treatment

A new drug, taken twice daily for one month, is an effective treatment for a certain disease. The drug now most commonly prescribed for the disease occasionally has serious side effects such as seizures; the new drug has side effects much more frequently, but the worst of them is mild nausea. Therefore, the new drug is clearly preferable as atreatment.

Which of the following, if true, most seriously weakens the argument?

A) It is possible that the drug now most commonly prescribed has side