Weight loss surgical procedures, in one form or another, achieve the desired effect of weight loss by altering absorption of fat, protein, and carbohydrates. This results in decreased total absorption of required calories.
An unintended consequence is the altered absorption of medications. Frequently I am asked about the specific medication. Usually the answer is vague since the information is limited on specific medications. If the desired effect is not achieved, then it is probably not being absorbed well. Specially, if the same dose of the same medication working well before surgery.
There is a summary article about the Theoretical absorption pattern of different weight loss surgical procedures.