Drug A acts on a receptor that's only expressed in the brain. Drug B is a blood pressure-lowering compound that cannot cross the blood-brain barrier. Is it more likely that Drug A causes a pharmacodynamic interaction on the action of Drug B, or Drug B to have a pharmacodynamic interaction on Drug A? Explain your thinking.

drug A is probably an antidepressant or antipsychotic drug. drug B lowers blood pressure.

Both are serious conditions, therefore, a doctor would recommend taking drug A first, drug b would interfere drug A's efficiency. dr. would recommend taking drug B at least 4 hrs. after drug A, so both drugs are effective for it's intended cause ...

I hope this makes sense, it's more about which condition is more serious that you would need to treat first ...