As it is with traditional pharmacies, on-line drugstores rely on prescriptions to be successful, since it is primarily prescriptions that attract the customers, who then also buy other health-related items.
As it is with traditional pharmacies, on-line drugstores rely on prescriptions to be successful
As with the case of traditional pharmacies, on-line drugstores rely on prescriptions to have success
As is the case with traditional pharmacies, prescriptions are the cornerstone of a successful on-line drugstore
As traditional pharmacies, so on-line drugstores rely on prescriptions to be successful
Like traditional pharmacies, the cornerstone of a successful on-line drugstore is prescriptions
"is/was the case" is a construction that is allowed to stand for the entire idea in the rest of the comparison; it does not require any sort of grammatical parallelism, so there's really no such thing as "omitted words" here.
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