Why is quality control necessary in a pharmacy?

Many medications have different possible dosing regimens which are up to a Doctor Who may prescribe a few, or a lot of pills. And, sometimes the pills can be prescribed in varying doses. Thus the pharmacies often purchase the pills in bulk and individually count and distribute the pills, and make custom labels for the containers which include the medication, quantity, and dosing recommendations.

Often handwritten prescriptions are difficult to read.

Some medications are toxic if taken in too high of a dose, or they can react with other medications (or even over the counter substances like alcohol).

At too low of a dose, they would be ineffective.

The pharmacist is the "last stop" essentially before the patient ingests the medications.

So, the pharmacist must make sure what is in the package is in fact what the doctor prescribed, and that they put the right medication at the right dose in the bottle.

They also should make sure the dose and dosing is "reasonable". If not, then they should double check the prescription, and perhaps even discuss it with the prescribing physician.

If the patient has filled, or regularly fills other medications at the same pharmacy, then the pharmacist should check for drug interactions, and possible contraindications of the medication. If the patient has any known alergies, then obviously those should be heeded too.

If there are strong, and potentially dangerous interactions with common substances such as alcohol (or sunlight), then that should be mentioned on the label, and the pharmacist should remind the patient.

Occasionally recalls happen, and potentially dangerous drugs will have to be removed from circulation as quickly as possible. They also have an expiration date. So, all medications should be tracked with source and batch number, and expiration date. And the pharmacist should be able to track who got what down to the end user.

Fortunately many of these dosing and interaction issues can be checked by modern computer systems, as well as inventory control.

Finally, like anything else, Pharmacists are potentially susceptible to fraud. They deal with a number of medications that are "controlled substances", and have significant "street value". There must be consistency checking in the inventory (this might also help with finding distribution errors).

Few medications are actually mixed by modern pharmacists (in the past they did that too).

The question was under Birth Control pills which are often distributed with fixed dosing regimens so that part can be dealt with at the factory. But, still a doctor should specify a quantity and number of refills. The pharmacist, of course, should be aware of the frequency of refills, and determine whether the patient is taking too few or too many.