No Drinking Before Bathing (飲酒後の入浴禁止 - Inshu go no nyūyoku)
The strict medical and safety prohibition against soaking in hot spring waters after drinking alcohol. Combining alcohol's vasodilatation with hot thermal waters can trigger dangerous blood pressure drops or heart attacks.
Onsen ryokans serve gorgeous feasts with sake, but hot springs post-alcohol lead to vascular accidents. Signs are posted at all entrances to warn guests, avoiding emergency medical interventions.
Always soak first, dry off, then enjoy your cold draft beer or sake. If you drink during your gorgeous multi-course dinner (kaiseki), sleep it off completely before attempting another hot soak.
Why is soaking in hot onsen waters strictly banned after drinking alcohol?