What goes on in a pharmacy behind the counter, is PRIVATE business. A pharmacy is not a public place...it is private property. When the theft LEAVES the premises, then a crime has been committed. This goes for shoplifting too. A store can choose to not press charges you know in some crimes as well.

It is up to the owner/owners to oversee its functions.

If you know of large amounts of controlled drugs LEAVING illegally the place of business...you see transactions happening in the parking lot, or see things after hours....then you can call police. Outside is a public venue. It is illegal to have unlabeled drugs on your person (without a legal prescription vial) and you can be arrested for that. Pharmacists may be excluded, if transfer paperwork is present, say moving a small item from one store to another.

Most likely for large problems the DEA would be the one to do it.
In robberies, the DEA is often one of the agencies involved in the investigation.

But if you just suspect things, with no evidence, you need to get EVIDENCE before you blow any whistles. You have to prove it was not an "error" or professional mistake. And corporate is the one to gather this evidence and decide to call the authorities. The drugs belong to corporate.

It can get pretty complex....but in 40 yrs I have NEVER seen or heard of police being involved other than in robberies at the counter (by criminals) or break ins after hours. We call the police on theft from the floor and shoplifting, or disorderly conduct. I've been the one called into problem places when the pharmacist is being removed from duty. This is usually done by surprise, so they don't know what is happening.
But many times I've been the one to come in after, and clean up so to speak. I've seen many things over the years. And it is not common or pleasant.