Critical Reasoning (CR) | Some doctors believe that a certain drug reduces the duration

Hi Experts GMATNinja GMATNinjaTwo mikemcgarry
Is below approach correct:
main conclusion:
the drug has no effect on the duration of vertigo
What leads to this conclusion:
a. Initially it was believed by few docs that certain drug reduces the duration of episodes of vertigo. This led to another intermediate claim that since the drug was introduced, the average duration of vertigo for people who suffer from it has decreased
b. Counter premise suggests that in spite of shortage of drug in past 3
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