There currently are pharmacies that are practicing therapeutic substitution when it comes to filling prescription medications. Therapeutic substitution happens when a drug that is considered to be therapeutically equivalent to a second drug, even though it might not be chemically equivalent to the prescribed drug, is given to the person filling the prescription without their permission or acknowledgment. That means that a pharmacy can change your prescribed medication to a different one without asking for your permission or letting you know beforehand. It can happen due to costs to the insurer, changes in insurance coverage, new drugs on the market that can treat the same condition, a medication is no longer covered by the insurance company, when your doctor wants you to switch to a drug that might be more effective, and when another drug is available at a lower cost. Pharmacies can also do a substitution if the drug you are prescribed is not on the pre-approved list that insurance will cover.
There are negative sides to therapeutic substitution because the new drug might not work as well or might differently that the one originally prescribed. It could have side effects that you are not aware of or might have a negative interaction with other prescriptions that you make take. The new drug might have a dosing schedule that could be inconvenient for you.
It is recommended to check the label of your medication after it is filled to make sure that it is the one that was originally prescribed. If you have questions or feel unsure about something, don't feel afraid to talk to your pharmacist.