“If I have a glass of alcohol, when should I take my metoprolol?”

Drinking alcohol increases cardiac irritability including atrial fibrillation and other arrythmias. It does this in a dose related fashion especially after three drinks or 90 grams of alcohol since it makes the RBCs so stiff they cannot pass through the capillary bed causing rouleaux formation (red cells stacking up like a roll of coins at the capillary entrance. The capillary is smaller than the red cell. As a result the body's total peripheral resistance increases and the blood pressure rises. So alcohol may defeat the reason you are taking a Beta-blocker whether for blood pressure or as an anti-arrthymic.