Why do some doctors not prefer the patient get generic brand of rx?

Effectiveness: Some times generic medication does not work as effectively as brand name.

Usually generic : Usually generics are fine. Most doctors try to avoid brand names to save their patients money. Some drugs are very difficult to make and even if they are available in generic form, they may not be as potent or consistently produced. Batches can differ in potency for example. For that reason some doctors insist on brand names only.

Many, if not most: Generics are exactly the same as brand name medication. One noteworthy exception is thyroxine, where the brand name replacement is more consistent and reliable. Drugs that are replacement hormones, such as thyroid, have a much more variable effect in your body with only the slightest variation in the medication, the fda tests by drug levels, not by physiologic action in your body, which can vary.