 13C-dextromethorphan breath test: Patients receive oral Alka-Seltzer Gold ASG; citric acid, potassium bicarbonate, and sodium bicarbonate in water, then, 15 minutes later, another ASG dose and oral 13C-dextromethorphan. Patients breathe into a bag 1-2 times, and the is bag sealed. 13CO_2 levels in the bags are measured. 