A randomized controlled trial published in JAMA measured this head-to-head. Calcium carbonate antacid had rapid onset — neutralizing acid within 30 minutes — but its duration was only about 60 minutes. OTC famotidine was the mirror image: delayed onset around 90 minutes, then duration of at least 540 minutes (about nine hours). An antacid is for heartburn you already feel; an H2 blocker is for coverage across a stretch of hours.

FDA is explicit: OTC proton pump inhibitors — Prilosec OTC, Nexium 24HR, Prevacid 24HR, Zegerid OTC — treat frequent heartburn (2+ days a week) and are “intended only for a 14-day course of treatment,” usable up to three times per year. PPIs are not intended for immediate relief and may take one to four days for full effect, which is why people wrongly conclude they “aren't working” and double up.

NIH's Office of Dietary Supplements states that gastric acid inhibitors can interfere with vitamin B12 absorption from food, potentially leading to deficiency. The drugs named include PPIs like omeprazole and lansoprazole, and H2 blockers like cimetidine. The mechanism: stomach acid is what frees B12 from the proteins it's bound to in food — and the flagged concern is prolonged use.

Cleveland Clinic lays out the timing, which explains most laxative misuse. Bulk-forming (fiber) laxatives take 12 hours to three days and are least likely to cause side effects. Osmotic laxatives take one to three days. Stimulant laxatives work in six to 12 hours. Most laxatives are meant for occasional or short-term constipation — taking stimulants longer than directed can cause loss of colon muscle tone.

Charcoal capsules are marketed for gas as if the science were closed. It isn't. In a study published in the American Journal of Gastroenterology, activated charcoal produced no significant reduction in the fecal release of sulfur-containing gases, and neither total gas nor abdominal symptoms were significantly influenced. The authors concluded commonly used doses do not appreciably reduce fecal gas.