Antacids, alginates, acid reducers, stomach coaters, anti-diarrhoeals and laxatives all sit on the same pharmacy shelf but work in very different ways. This guide explains which one fits heartburn, reflux, an ulcer, diarrhoea or constipation, how to time them with other medicines, and which symptoms need a doctor rather than another packet.
The short answer
- Occasional heartburn: an antacid or alginate (Gaviscon) after meals works within minutes.
- Frequent reflux: an acid reducer, either an H2 blocker (famotidine) or a proton pump inhibitor (omeprazole, esomeprazole, pantoprazole) taken 30–60 minutes before breakfast.
- Diarrhoea: fluids first; loperamide (Imodium) for adults without fever or blood. Constipation: fibre and fluids, then macrogol or a stimulant laxative.
Which stomach medicine does what?
| Group | Examples (brands) | How it works | Works in |
|---|---|---|---|
| Antacids | Calcium carbonate (Tums, Rennie), magnesium/aluminium (Mylanta) | Neutralise acid already in the stomach | Minutes; lasts 1–2 hours |
| Alginates | Gaviscon | Form a foam “raft” on top of stomach contents that blocks reflux | Minutes; up to 4 hours |
| H2 blockers | Famotidine (Pepcid) | Reduce acid production | About 1 hour; lasts 10–12 hours |
| Proton pump inhibitors (PPIs) | Omeprazole (Losec), esomeprazole (Nexium), pantoprazole (Somac, Protonix), lansoprazole | Switch off the acid pumps more completely | Full effect after 2–4 days of daily use |
| Mucosal protectant | Sucralfate (Carafate, Carafate liquid) | Coats ulcers with a protective layer | Prescribed for ulcers |
| Bismuth | Pepto-Bismol | Soothes, mild anti-diarrhoeal; part of some H. pylori treatments | 30–60 minutes |
| Anti-diarrhoeal | Loperamide (Imodium, Gastro-Stop) | Slows gut movement | 1–3 hours |
| Antispasmodics | Hyoscine butylbromide (Buscopan), mebeverine (Colofac) | Relax gut muscle cramps | 15–30 minutes |
| Anti-wind | Simeticone (Gas-X, Infacol, De-Gas) | Breaks up gas bubbles | Minutes |
Ranitidine (Zantac), once the best-known H2 blocker, was withdrawn worldwide in 2019–2020 after an impurity called NDMA was found. Famotidine is the usual replacement.
Heartburn and reflux: antacid, Gaviscon or acid reducer?
For heartburn after a large or spicy meal now and then, an antacid or alginate is enough. Gaviscon’s raft is particularly useful for night-time reflux and is safe in pregnancy. If symptoms come back several times a week, a regular acid reducer is more effective. PPIs work best taken 30–60 minutes before the first meal of the day, because the pumps they block are switched on by food.
A typical approach is a 4-week course of a PPI at the standard dose, then stepping down to the lowest dose that controls symptoms, or to taking it only when needed. Long-term daily PPI use is appropriate for some people (for example with Barrett’s oesophagus, or alongside NSAIDs or steroids), but it is linked to lower magnesium and B12 levels, a higher risk of some gut infections and possibly fractures, so it is worth reviewing once a year.
Timing with other medicines: antacids and Gaviscon bind or affect the absorption of many medicines, including some antibiotics (doxycycline, ciprofloxacin), iron, levothyroxine and HIV medicines. Keep them about 2 hours apart. Sucralfate has the same problem; see how to take famotidine and sucralfate together.
Ulcers and H. pylori tests
Helicobacter pylori is a bacterium that causes most stomach and duodenal ulcers not caused by NSAIDs. It is diagnosed with a breath test, a stool antigen test or a biopsy. Medicines that suppress the bacteria can make these tests falsely negative, so before testing you usually need to stop:
- PPIs: 2 weeks before the test.
- Antibiotics and bismuth: 4 weeks before.
- H2 blockers: some labs ask for 1–2 days; follow the lab’s instructions.
Antacids and alginates can usually be used for symptom relief during this time. Read does Gaviscon affect an H. pylori test? for the details.
Diarrhoea and gastro
Most acute diarrhoea is caused by a virus and settles within a few days. The main risk is dehydration, so oral rehydration solution (Hydralyte, Dioralyte, Pedialyte) matters more than any medicine. For adults who need to travel or work, loperamide reduces the number of trips to the toilet: usually 4 mg (two capsules) to start, then 2 mg after each loose stool, without exceeding the label maximum (6–8 capsules a day over the counter, depending on the product and country).
Don’t use loperamide if you have a fever, blood or mucus in the stool, severe abdominal pain, after recent antibiotics (possible C. difficile), or for children unless a doctor advises it. Very high doses have caused dangerous heart rhythm problems. Avoid ibuprofen during gastro, because NSAIDs plus dehydration can damage the kidneys. Paracetamol is the better choice for aches and fever.
If you need to identify capsules from a pack or pill organiser, see what Imodium (loperamide) pills look like.
Constipation: which laxative?
| Type | Examples | Onset | Best for |
|---|---|---|---|
| Bulk-forming | Psyllium (Metamucil, Fybogel), sterculia | 2–3 days | Mild constipation, low-fibre diet; needs plenty of water |
| Osmotic: macrogol | Movicol, OsmoLax, MiraLAX | 1–3 days | Most adults and children; well tolerated |
| Osmotic: lactulose | Duphalac, Actilax, generic | Up to 2 days | Gentle; sometimes used in liver disease; can cause wind |
| Stimulant | Senna (Senokot), bisacodyl (Dulcolax) | 6–12 hours | Opioid constipation, short-term relief |
| Softener | Docusate (Coloxyl, Colace) | 1–3 days | Hard stools; weak on its own |
| Suppositories / enemas | Glycerin, bisacodyl, Microlax | 15–60 minutes | Rapid relief |
Lactulose doses, bottle sizes and taste tips are in lactulose bottle sizes and doses. Laxatives around operations are covered in Colace before and after surgery.
Gut symptoms that need a doctor
See a doctor rather than treating yourself if you have difficulty or pain swallowing, unintended weight loss, vomiting that persists or contains blood, black or bloody stools, a lump in the abdomen, anaemia, or new indigestion that doesn’t settle at age 55 or over. Also seek advice for diarrhoea lasting more than a few days, signs of dehydration (very little urine, dizziness), or constipation with severe pain, vomiting or a swollen belly.
Articles in this guide
Each article answers one specific question in depth.
- Does Gaviscon Affect an H. pylori Test?PPIs and antibiotics can cause false-negative H. pylori breath and stool tests; Gaviscon and plain antacids usually don't. What to stop and when.
- How to Take Famotidine and Sucralfate TogetherSucralfate binds other medicines, so timing matters. A simple daily schedule for famotidine and sucralfate, plus tips for liquid and tablets.
- Lactulose Bottle Sizes, Doses and How to Take ItLactulose solution (about 3.3 g per 5 mL) comes in bottles from roughly 200 mL to 1 litre. Doses for constipation, taste tips and how long it takes.
- What Do Imodium (Loperamide) Pills Look Like?Loperamide 2 mg comes as green-and-grey or green-and-white capsules, caplets and melt tablets. How to identify pills safely, and dose limits.
Frequently asked questions about stomach medicines
Is Nurofen good for gastro?
No. Ibuprofen can irritate an already upset stomach, and combined with dehydration it can harm the kidneys. Use paracetamol for fever and aches during gastro, and focus on fluids.
Can I take Gaviscon and omeprazole together?
Yes. They are often used together: omeprazole for daily control, Gaviscon for breakthrough symptoms. Leave a gap of about 2 hours between Gaviscon and your other medicines.
How long does Gas-X (simeticone) take to work?
Simeticone acts in the gut within minutes to break up bubbles, so relief, if it comes, is usually quick. Evidence for it is modest; if bloating is frequent, a doctor can look for causes such as food intolerance or IBS.
Can I take Buscopan with paracetamol?
Yes. They work differently and are often combined for cramping abdominal or period pain. See Buscopan and paracetamol.
Sources and further reading
Pain reliefHow paracetamol (Panadol), ibuprofen (Nurofen), aspirin and diclofenac differ, safe doses, combining them, and who…
Combining medicinesWhich medicines clash and why: duplicate ingredients, NSAID combinations, sedatives, alcohol and blood pressure…
Surgery & proceduresWhich medicines to stop before surgery, what is usually fine, fasting rules, and pain relief, laxatives and…