The pharmacist reached for two boxes behind the counter, turning them so the labels faced the customer. "These are the most common kinds — calcium carbonate and magnesium hydroxide. Both help with heartburn, but they work in different ways," she explained, setting them side by side.

"So, what's the main difference?" the customer asked, picking up the box of calcium carbonate. The pharmacist nodded. "Well, calcium carbonate starts to work quickly because it reacts with stomach acid fast. It usually gives longer-lasting relief, but, if you take too much, it might cause constipation. On the plus side, a lot of people like that it's been around a long time."

She tapped the box of magnesium hydroxide. "This one is gentler on the stomach for some people, and it can help if you're feeling a bit blocked up, since it might cause diarrhea instead. The relief might not last as long, though, so you'd sometimes need to take it more often. And, people with kidney problems should talk to their doctor first."

The pharmacy was busy, with customers peering at shelves and someone calling out for tissues. The pharmacist paused for a moment, then said, "If you want something fast and steady, calcium carbonate is trusted. But if you need something lighter, magnesium hydroxide isn't a bad choice." She slid both boxes forward, just as a small bell rang near the door.