Common GI drug classes and safe prescribing

01Purpose and principlesWhat the treatment does and how it fits into care.

Gastrointestinal prescribing is deceptively vulnerable to indication drift. A PPI started with critical illness, an antiemetic added during chemotherapy or a laxative issued with one opioid may remain on repeat after the original context disappears. Medication reconciliation should ask what each agent is treating now, whether it works, and which harm signals are accumulating.

Mechanism guides safer selection. Dopamine antagonists may help nausea or gastric delay but cause extrapyramidal or cardiac effects; 5-HT3 antagonists are effective in selected emetogenic settings yet prolong QT and constipate. Anticholinergic antispasmodics can worsen retention, glaucoma, cognition and constipation. Diarrhoea may need rehydration and diagnosis rather than slowed transit.

The current BNF, product information, MHRA safety updates and local formulary are the prescribing source of truth. Dose, licensed duration and contraindications differ by agent, indication, age, organ function and route; a textbook should teach the decision architecture and high-risk exceptions, not replace point-of-prescription checking.

Key points

Treat the cause and dangerous alternatives before suppressing a symptom: antiemetics, antidiarrhoeals, antispasmodics and laxatives can obscure obstruction, sepsis, bleeding or inflammatory disease.
For every GI drug, write the indication, intended duration, review or stop point, renal and hepatic considerations, pregnancy context, interactions and what response would justify continuation.
Proton-pump inhibitors are effective for acid-mediated disease and gastroprotection when risk warrants it, but lowest effective intensity and review reduce unnecessary long-term exposure and rebound-driven continuation.
Metoclopramide is restricted to short-term use because of neurological harm; the MHRA adult maximum is 30 mg daily and treatment should not usually exceed five days.
Domperidone carries cardiac risk, should use the lowest effective dose for the shortest duration, and is contraindicated in cardiac conduction risk, important interacting medicines and, since 2026, phaeochromocytoma.
Choose laxatives from stool pattern and mechanism after excluding obstruction: bulk-forming agents need adequate fluid, osmotics retain water, and stimulants enhance propulsion.
Loperamide is inappropriate in ileus, toxic megacolon risk or severe bloody inflammatory or infectious diarrhoea; excessive dosing can cause fatal ventricular arrhythmia.
Bile acid sequestrants can improve confirmed or strongly suspected bile acid diarrhoea but bind other medicines and may impair fat-soluble vitamin absorption, so timing and follow-up matter.

02Indications, selection and cautionsWho may benefit, who needs urgent treatment and important alternatives.

Acid indication

Documented reflux oesophagitis, peptic ulcer treatment, Helicobacter pylori regimen or justified antiplatelet and NSAID gastroprotection supports PPI use with an endpoint.

Medicine-induced nausea

Opioids, antibiotics, dopamine agonists, cytotoxics and metabolic treatments can cause nausea, but raised intracranial pressure, obstruction and sepsis must remain in the differential.

Constipation phenotype

Hard stool, infrequent passage, straining, incomplete evacuation, overflow and opioid exposure point toward different combinations and sometimes rectal examination or imaging.

Inflammatory diarrhoea

Blood, fever, severe pain, systemic toxicity, recent antibiotics or immunosuppression argues against reflex antimotility treatment and toward urgent cause assessment.

Bile acid pattern

Watery urgency after terminal ileal disease or resection, cholecystectomy or unexplained chronic diarrhoea may indicate bile acid loss, ideally supported by the commissioned diagnostic pathway.

Cumulative risk

Older age, frailty, renal or hepatic impairment, multiple QT drugs, electrolyte depletion and anticholinergic burden amplify adverse effects from otherwise ordinary prescriptions.

Red flags requiring action

Persistent vomiting with distension, severe pain, absent stool or flatus, dehydration or peritonism requires urgent obstruction or acute-abdomen assessment before prokinetics, laxatives or oral medicines.
Haematemesis, melaena, haemodynamic compromise or progressive dysphagia needs urgent diagnostic management and must not be masked by empirical acid suppression alone.
Bloody diarrhoea with fever, severe tenderness, dilatation or systemic toxicity raises infectious colitis, inflammatory flare or toxic megacolon risk; avoid loperamide pending expert assessment.
Syncope, palpitations, seizure or new ventricular ectopy while taking domperidone, ondansetron, high-dose loperamide or interacting QT medicines requires immediate ECG and electrolyte evaluation.
Oculogyric crisis, dystonia, akathisia or parkinsonism after metoclopramide should prompt drug cessation and urgent clinical review rather than another antiemetic dose.

03Assessment before treatmentTests and checks that guide safe selection.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Structured medication reconciliationFirst step
Why
Identify active indications, duplicate mechanisms, interactions, self-medication and prescriptions that have outlived their purpose.
Interpretation and limitations
Include OTC antacids, laxatives, antihistamines, herbal products and alcohol; absence of an indication or review date is a prescribing problem, not evidence of continued need.
02
ECG and QT-risk assessment
Why
Reduce pro-arrhythmic harm before or during selected antiemetic and high-dose antimotility treatment.
Interpretation and limitations
Consider baseline QTc, bradycardia, structural disease, congenital history, potassium and magnesium, plus all interacting drugs; a single normal ECG cannot neutralise a changing risk profile.
03
Renal hepatic and electrolyte profile
Why
Guide agent choice and reveal consequences that increase toxicity.
Interpretation and limitations
Renal impairment changes magnesium-containing laxative and medicine exposure, liver disease affects metabolism, and low potassium or magnesium magnifies cardiac risk.
04
Pregnancy and lactation check
Why
Ensure symptom treatment and diagnostic delay are considered in the correct reproductive context.
Interpretation and limitations
Use current BNF and obstetric guidance for the exact agent and gestation; pregnancy does not make vomiting, pain or bleeding automatically benign.
05
Cause-specific gastrointestinal assessment
Why
Establish whether endoscopy, stool testing, imaging or inflammatory investigation is needed before symptomatic therapy.
Interpretation and limitations
Alarm features, duration, surgery, travel, antibiotic exposure, weight loss and examination should determine tests; response to a medicine does not retrospectively prove the presumed diagnosis.
06
Therapeutic response review
Why
Decide continuation, step-down, switch or cessation against a pre-agreed endpoint.
Interpretation and limitations
Measure the relevant outcome such as reflux frequency, vomiting, complete spontaneous bowel movements or stool urgency alongside adverse effects and adherence.

04Treatment approachPreparation, options, escalation and aftercare.

01CHECKBefore prescribing a GI symptom drugFirst stepA patient requests treatment for pain, nausea, reflux, constipation or diarrhoea.

1Define symptom chronology, likely mechanism, red flags, comorbidity, pregnancy possibility and current prescription or non-prescription exposure.
2Examine and investigate enough to exclude an acute abdomen, bleeding, severe infection or metabolic emergency before suppressing the signal.
3Choose the narrowest effective agent using the current BNF, SmPC and local formulary, with organ-function and interaction checks.
4Write indication, course, response target, adverse-effect advice and a dated stop or review instruction.

02ACIDPPI initiation and reviewAcid-mediated disease or a validated gastroprotection indication is likely.

1Match standard or higher-intensity acid suppression to the indication and arrange investigation for dysphagia, bleeding, anaemia, weight loss or persistent vomiting.
2Check interacting medicines and whether Helicobacter pylori testing timing, eradication therapy or endoscopic follow-up changes the plan.
3At review, step down or stop when the treatment course is complete, warning that transient rebound symptoms can occur.
4For continuing long-term therapy, periodically reconfirm benefit and assess magnesium, fracture, infection or nutrient concerns according to individual risk.

03NAUSEAMechanism-based antiemetic selectionNausea or vomiting persists after urgent surgical, neurological and metabolic causes are addressed.

1Identify likely vestibular, gastric-stasis, chemical, treatment-related or obstructive mechanism and correct hydration or electrolyte disturbance.
2Review QT, parkinsonism, age, bowel function, pregnancy, hepatic function and interacting medicines before choosing a class.
3Use the lowest effective licensed regimen for a defined brief period, respecting metoclopramide and domperidone regulatory restrictions.
4If symptoms persist, revisit diagnosis and specialist options rather than layering several dopamine, serotonin and anticholinergic agents blindly.

04BOWELConstipation treatment ladderConstipation is confirmed without obstruction, toxic megacolon or faecal impaction requiring a different approach.

1Address fluid access, mobility, toileting, fibre suitability and causative medicines, including prophylaxis when opioids are started.
2Select bulk-forming, osmotic or stimulant therapy from stool consistency and propulsion, titrating one coherent regimen before adding complexity.
3Assess rectal loading or overflow when symptoms and stool chart conflict, using rectal measures only with consent and clinical indication.
4For refractory or opioid-induced disease, verify adherence and diagnosis before specialist secretagogues, prokinetics or peripheral opioid antagonists.

05LOOSESafe diarrhoea controlFrequent loose stool causes disability but dangerous inflammatory, infectious or obstructive features are absent.

1Prioritise oral rehydration and identify medicines, recent antibiotics, travel, surgery, malabsorption and inflammatory features.
2Use loperamide only when slowing transit is appropriate, within licensed guidance unless a specialist intestinal-failure protocol explicitly governs higher dosing.
3Investigate chronic watery diarrhoea for bile acid loss and other causes rather than leaving indefinite empirical treatment unreviewed.
4EscalationMonitor hydration, abdominal distension, stool response and cardiac risk, stopping and escalating if pain, blood, fever or ileus appears.

05Regimens, contraindications and interactionsTreatment details and the circumstances that modify them.

Suppress gastric acid for reflux oesophagitis, peptic ulcer treatment and selected evidence-based gastroprotection.

Proton-pump inhibitor

Use the BNF regimen matching the licensed indication, then step down to the lowest effective intensity or stop after the planned course.

Check interactions and ongoing indication; prolonged therapy is associated with hypomagnesaemia and other infection, bone or nutrient concerns, while abrupt cessation can cause rebound symptoms.

Provide short-term dopamine-antagonist antiemetic and prokinetic action in an appropriate non-obstructed clinical setting.

Metoclopramide

For adults, do not exceed 10 mg up to three times daily or 30 mg daily, and usually limit treatment to five days.

Avoid in Parkinson's disease, gastrointestinal obstruction or bleeding and previous tardive dyskinesia; reduce as required for organ dysfunction and stop for extrapyramidal reactions.

Treat selected nausea and vomiting when its peripheral dopamine-antagonist profile offers a favourable balance.

Domperidone

Use 10 mg up to three times daily, with 30 mg daily the adult maximum and treatment ordinarily no longer than one week.

Avoid in significant cardiac disease, prolonged QT, electrolyte depletion, potent CYP3A4 or QT interactions, hepatic impairment and phaeochromocytoma; reassess rather than extending routine courses.

Block 5-HT3-mediated nausea and vomiting, particularly in postoperative or cancer-treatment settings.

Ondansetron

Use the lowest effective licensed regimen for the specific emetogenic indication, route and organ function, following current BNF guidance.

Check QT and electrolyte risk, constipation or ileus, hepatic impairment and serotonergic combinations; it is not an indiscriminate treatment for undiagnosed persistent vomiting.

Soften stool through osmotic water retention or improve propulsion in established constipation and opioid-related bowel dysfunction.

Macrogol or stimulant laxative

Select and titrate the BNF adult regimen to stool consistency and frequency, adding complementary mechanisms only when the response remains inadequate.

Exclude obstruction and impaction, account for fluid and electrolyte vulnerability, and recognise that bulk-forming products can worsen symptoms when fluid intake or luminal patency is poor.

Reduce motility and secretion for appropriate non-inflammatory diarrhoea and selected high-output states.

Loperamide

Use the licensed short-term or chronic-diarrhoea schedule and stop at the stated maximum unless an intestinal-failure specialist directs monitored off-label dosing.

Avoid ileus, abdominal distension, severe colitis and dysentery; overdose or misuse causes QT and ventricular arrhythmia, especially with interactions or low electrolytes.

Bind luminal bile acids that drive watery secretion after ileal dysfunction or in primary bile acid diarrhoea.

Bile acid sequestrant

Start the formulary product at a tolerated scheduled dose and titrate to stool response within the commissioned bile acid diarrhoea pathway.

Separate other medicines by the product-specific interval, review triglycerides, constipation and fat-soluble vitamins, and note that extensive ileal loss may worsen fat malabsorption.

Reduce smooth-muscle spasm in selected functional bowel symptoms after alarm causes are excluded.

Anticholinergic antispasmodic

Use a short BNF-guided trial at the lowest effective dose with a documented symptom target and stop date.

Dry mouth, blurred vision, urinary retention, constipation, glaucoma, tachycardia and cognitive effects are important, particularly in frailty or an already high anticholinergic burden.

06Complications, monitoring and follow-upAdverse effects, response and longer-term review.

At every repeat review, confirm the active indication, benefit, actual dose, duration, adherence, adverse effects and whether a trial reduction is appropriate.
For QT-risk antiemetics or specialist high-dose loperamide, reassess ECG need, potassium, magnesium, renal function and newly added interacting medicines.
During long-term PPI treatment, reconsider gastroprotection risk and investigate magnesium or other complications according to symptoms, duration and concomitant diuretics or digoxin.
Use stool frequency and form, abdominal pain, distension, bleeding and hydration to judge laxative or antidiarrhoeal safety rather than counting tablets alone.
Check neurological symptoms promptly during dopamine-antagonist use and document that metoclopramide and domperidone durations remain within current restrictions.
When bile acid sequestrants continue, monitor adherence, spacing from critical medicines, constipation, triglycerides and relevant fat-soluble vitamin status.

07Special situationsVariants, exceptions and circumstances that change the usual approach.

A stop date is therapeutic

Time-limited prescribing prevents a transient postoperative or acute-illness indication becoming years of unexamined exposure on repeat medication.

Mechanisms can collide

Ondansetron may relieve nausea while worsening constipation; anticholinergics can ease spasm yet aggravate retention, cognition and slow transit.

QT risk is cumulative

A modestly pro-arrhythmic drug becomes more dangerous beside bradycardia, structural heart disease, low minerals and several interacting prescriptions.

PPI response proves little

Improvement in dyspepsia does not exclude gastric cancer or eosinophilic disease when alarm features remain, because acid suppression is non-specific.

Binding changes exposure

Bile acid sequestrants can reduce absorption of narrow-therapeutic-index and routine medicines alike, making product-specific dose separation part of the prescription.

Symptom suppression has a ceiling

Failure of a rational short trial is a prompt to re-evaluate diagnosis, delivery and adherence, not automatically to stack more classes.

08Common pitfallsFrequent interpretation and management errors.

01
Do not renew a PPI indefinitely without recording the original indication, current gastroprotection risk and an attempted lowest effective plan.
02
Do not prescribe metoclopramide beyond five days routinely or overlook dystonia, akathisia and parkinsonism as drug effects.
03
Do not use domperidone without checking cardiac, QT, interaction, hepatic and phaeochromocytoma contraindications against current regulatory advice.
04
Do not treat constipation with bulk fibre when obstruction, impaction, severe dehydration or inability to drink makes expansion hazardous.
05
Do not slow bloody febrile diarrhoea or a distended toxic bowel with loperamide before urgent diagnostic assessment.
06
Do not give bile acid sequestrants simultaneously with the rest of the medication round or ignore fat-soluble vitamin and triglyceride consequences.
07
Do not select an antiemetic solely from habit; match likely mechanism while accounting for bowel function, Parkinsonism, QT and pregnancy.
08
Do not assume an OTC GI product is harmless, because antacids, laxatives and anticholinergics contribute interactions, electrolyte change and cumulative burden.