Tums, an antacid made of calcium carbonate, starts working almost immediately after swallowing by neutralizing stomach acid. Most people feel relief within seconds to a few minutes. The effect is usually fast because it is a direct chemical reaction between Tums and stomach acid. However, the full relief effect is generally felt within about 5 minutes and it can last for about 20-30 minutes. Antacids like Tums relieve heartburn and indigestion quickly, typically within minutes, but they are short-acting compared to other acid reducers. For best effect, they are often taken after meals or when heartburn symptoms begin.