I personally have not heard of this. I think psychiatrists walk a fine line though with many psychiatric patients stepping over into the area of unprescribed drugs (either things like Xanax or pain killers). This often goes hand in hand with mental health. Unfortunately. Perhaps he wanted to be sure you were not using other substances? Maybe someone else here has more insight. Minipress is taken for blood pressure or do they use it like they do beta blockers for acute anxiety situations? I could be wrong . . . maybe they can check Effexor levels and it is excreted through kidneys so I am sure they can. Would he be wondering if you are taking your Effexor? Just seems an odd way to make a dosage adjustment. :>) Anyway, hopefully someone else will weigh in.
First, I have no idea what minipress is so I can't comment about it. If it is a drug for blood pressure, and not for anxiety, blood pressure meds can have problems. But my understanding of these types of meds is that the only reason a psychiatrist or any doc might test you is to make sure you're absorbing the drug, but to do that you'd do a blood test, not a urine test, in my experience -- every time you have a physical your blood work will show your levels of meds you're taking. The would be true of Effexor, but again, I don't know anything about minipress. I would ask the psychiatrist why he did that and if you don't like the answer, find a new one -- this stuff is hard enough without having a doc you can't trust to tell you in advance what he's doing and why. I will only add, there is one class of psychiatrist that practices medicine, and they call themselves functional physicians. They are very expensive, don't take insurance, and will test patients for diseases before concluding a patient has a mental illness. I doubt you're seeing one of those, because the first thing they do is give you a battery of very expensive tests of your hormonal levels, blood sugar, etc. Otherwise, psychiatrists don't practice medicine, they usually nowadays don't do therapy, and are mostly there as experienced prescribers of difficult meds to take that affect brain neurotransmitters (and the salesforce for medication).