Reasoning GP · Algorithm · 1-page summary
Unintentional weight loss in adults
For: adults with unexplained, unintentional weight loss. Not for: children (paediatric pathways) or planned weight loss; known eating disorder or malnutrition → use those protocols.
Core rule: confirm it is unintentional and measure it. Most is not cancer, but if weight keeps falling with normal tests, reassess and use the non-specific symptoms pathway.
0Stop — act now — before the full work-up
- Physiological instability, haemodynamic instability from GI bleeding, suspected sepsis (NG253) or acute decompensation → 999
- Severe malnutrition (do not rely on BMI alone) or unable to eat or drink → Same-day medical assessment
- Possible eating disorder with syncope, severe bradycardia, hypotension or postural drop, hypothermia, ECG abnormality, severe electrolyte disturbance, dehydration, rapid loss, purging or high suicide risk → Same-day medical assessment (MEED)
- Alcohol excess with confusion, ataxia or eye-movement signs (Wernicke's) → Emergency hospital transfer; parenteral thiamine (e.g. Pabrinex) first if available, without delaying transfer — oral is not adequate.
1Triage — confirm and quantify
| Finding | Action |
|---|---|
| Intentional? | Diet, exercise, GLP-1 agonist, new drug? Disordered pattern → eating-disorder screen. |
| No past weight | Weigh now; use belt or clothing size change and family observations. |
| 60+, >5% in 6 mo | Look for other features, then offer urgent investigation, a suspected cancer pathway or the non-specific symptoms pathway (NG12). |
| Malnutrition | >10% in 3–6 months, or >5% with BMI <20 (CG32) → MUST and nutrition plan. |
| 40+, ever smoked | Weight loss alone → CXR within 2 weeks (NG12) |
2Screen — NG12 (updated April 2026); one row is enough
- Dysphagia (any age); 55+ with upper abdominal pain, reflux or dyspepsia → Suspected cancer pathway
- Lower GI (40+ with abdominal pain; <50 with rectal bleeding; 50+; bowel change, iron deficiency, abdominal mass) → FIT; ≥10 µg Hb/g → Suspected cancer pathway. Rectal mass or high concern → refer without waiting for FIT.
- Haemoptysis, 40+ → Suspected cancer pathway. 40+ with cough, fatigue, breathlessness, chest pain or appetite loss → CXR within 2 weeks
- Jaundice, 40+ → Suspected cancer pathway. 60+ with diarrhoea, back or abdominal pain, nausea, vomiting, constipation or new diabetes → consider urgent direct-access CT.
- Visible haematuria, 45+ (urology); postmenopausal bleeding, ascites or pelvic mass (gynaecology) → Suspected cancer pathway
- Haematology: concerning nodes → lymphoma criteria; unexplained bruising or bleeding → FBC within 48 h; 60+ persistent bone pain or unexplained fracture → myeloma screen incl. SPEP and serum free light chains.
3Assess — guided by history and examination
☐ Weight, BMI, frailty ☐ Nodes (incl. supraclavicular), thyroid, chest, abdomen, teeth ☐ Rectal, breast, testes, pelvis if indicated ☐ Mood, suicide risk
☐ FBC, U&E, LFTs, albumin, calcium, CRP/ESR, HbA1c, TFTs, urine dipstick. Selectively: coeliac serology (eating gluten), ferritin, B12/folate, HIV, CXR. Not routinely: CEA, CA-125, CA 19-9, AFP; PSA on age alone.
4Classify — only if the screen is negative
- Malignancy — older age, alarm features → NG12 (block 2).
- Appetite kept — hyperthyroidism, uncontrolled diabetes, coeliac, IBD, pancreatic insufficiency.
- Organic — TB, HIV, COPD, heart failure, CKD, cirrhosis, Addison's.
- Mental health — depression (common in older people), dementia, eating disorder, alcohol.
- Drugs, social — GLP-1, SGLT2, metformin, SSRIs, digoxin; poverty, isolation, teeth.
- Uncertain — say so; review in 2–4 weeks; still losing → non-specific symptoms pathway.
5Treat today — doses: NICE/SPC, BNF, local formulary
- MUST ≥2: nutrition plan, food first, consider dietitian; supplements only if clinically indicated.
- Hyperthyroidism: carbimazole only once confirmed; sore throat or fever → urgent FBC.
- Low mood: assess first; mirtazapine may suit depression with poor appetite, not as an appetite stimulant.
- Alcohol: AUDIT-C; oral thiamine if malnourished or at risk. Review appetite-suppressing drugs.
6Refer routinely
- Stable eating disorder → community eating-disorder service (NG69); BEAT.
- Abnormal result → the matching specialty pathway.
- Food insecurity, isolation → social prescribing, food banks; dentist.
7Never reassure by
- Normal first-line bloods while the weight keeps falling.
- BMI alone, in malnutrition or an eating disorder.
- Young age: dysphagia and postmenopausal bleeding need referral at any age.
- A FIT below 10 µg Hb/g when symptoms persist.
- A negative electrophoresis: it misses light-chain myeloma.
- "Depression" or "old age" before organic causes are assessed.
8Safety-net — say it, then document it
999 “Call 999 if you vomit blood, pass black stools, cough up blood and feel faint, collapse or become very confused.” Same day: “Contact us today if the weight loss speeds up, you cannot eat or drink, food sticks when you swallow, you cough up blood, bleed from the back passage or turn yellow.”
Review in 2–4 weeks if loss continues, tests are abnormal or pending, concern persists, nutritional risk is significant, or frail or vulnerable.
✓Safe to leave primary care only if — all five, documented
No block 0–2 row applies
Weight measured; % loss and time course recorded
First-line tests done or booked
MUST, mood, alcohol, drugs and social needs reviewed
2–4 week review booked; safety-net given
Summary of the full Reasoning GP unintentional weight loss algorithm, which holds the detail, examination, tests and evidence. Sources: NICE NG12 (updated April 2026), NICE FIT guidance, NG69, CG32, CG100, NG253; RCPsych MEED (2022); BAPEN MUST; BSG coeliac guidance. Clinical reference — follow local cancer, FIT and eating-disorder pathways and check doses: NICE/SPC, BNF.