Thickened fluids, IDDSI levels & oral nutrition support
What thickeners and oral nutritional supplements (ONS) are for, how the IDDSI 0–7 framework works, and how to use them safely. A plain-English guide — always under the supervision of a speech and language therapist (SLT) or dietitian.
Thickeners and oral nutritional supplements are not ordinary lifestyle products. The right fluid thickness is prescribed for a specific person by a speech and language therapist (SLT) after a swallow assessment; nutritional supplements are recommended by a dietitian or GP. Giving a drink that is too thin — or too thick — to someone with swallowing difficulty (dysphagia) can cause choking or send fluid into the lungs. This page explains the categories and the standards so you can ask informed questions. It is education, not a substitute for the assessment. If you have concerns about someone's swallowing, eating or weight, speak to your GP, pharmacist or the NHS.
The IDDSI framework: drinks (Levels 0–4)
IDDSI — the International Dysphagia Diet Standardisation Initiative — replaced the old 'syrup / custard / pudding' terms with eight numbered, colour-coded levels used worldwide. Drinks sit across Levels 0–4; texture-modified foods across Levels 3–7. The exact level a person needs is set by their SLT — never guessed. Thickness is checked with the simple IDDSI Flow Test: how much liquid is left in a 10 ml syringe after 10 seconds.
| IDDSI level | Old name | What it's like | Flow Test result |
|---|---|---|---|
| 0 — Thin | Thin | Flows like water; ordinary drinks | Less than 1 ml left after 10s |
| 1 — Slightly thick | (none) | Slightly thicker than water; anti-reflux / infant feeds | 1–4 ml left after 10s |
| 2 — Mildly thick | Nectar / Syrup | Sippable but slower; effort to suck through a straw | 4–8 ml left after 10s |
| 3 — Moderately thick | Honey / Custard | Drunk from a cup or taken with a spoon; pours slowly | 8 ml or more left after 10s |
| 4 — Extremely thick | Pudding | Doesn't flow; eaten with a spoon, holds its shape | Won't flow — use fork-drip / spoon-tilt test |
Drink levels and the 10 ml syringe Flow Test, per the IDDSI v2.0 framework. Levels overlap at 3 and 4 because some foods and drinks meet at that consistency.
What are oral nutritional supplements (ONS)?
Oral nutritional supplements are nutritionally complete or supplementary drinks, puddings or powders designed to top up energy, protein, vitamins and minerals when ordinary food and drink aren't enough. In the NHS they are classed as 'medical foods' and are usually recommended after a malnutrition screen (commonly the 'MUST' tool — a score of 2 or more flags high risk).
They're typically considered when someone has disease-related malnutrition, poor appetite, difficulty swallowing, or higher needs (for example during recovery, cancer treatment, or after major surgery). ONS come in different formats — milk-style, juice-style, yoghurt-style, savoury, and powders to fortify ordinary food — and some are pre-thickened for people with dysphagia so they meet a set IDDSI level straight from the carton.
Crucially, ONS are a step in a wider plan, not a first move. UK dietitians follow a 'food first' approach — enriching everyday meals with full-fat milk, cheese, butter and snacks before, or alongside, prescribed supplements.
Using thickeners and ONS safely — what good practice looks like
If you're caring for someone using thickened fluids or supplements, these are the things a well-run setup gets right. Use them as questions for the SLT, dietitian, GP or pharmacist.
A named IDDSI level — written down
The person should have a specific prescribed level (e.g. 'Level 2 — Mildly thick'). It should be recorded where everyone caring for them can see it, not passed on by word of mouth.
The right measuring scoop, every time
Powder thickeners are measured by the scoop that comes with that specific product — scoops are not interchangeable between brands. Too little risks the wrong level; too much can over-thicken and cause discomfort or refusal.
Wait, then re-check
Many thickeners keep thickening for a few minutes after mixing. Following the product's standing time and re-checking with the Flow Test avoids a drink that's thicker than intended by the time it's offered.
Hydration is watched, not assumed
People on thickened fluids often drink less because it's harder work — raising the risk of dehydration. A care plan should track fluid intake and flag any drop, especially in hot weather or illness.
Medicines reviewed for swallowing
Some tablets are hard to take with thickened fluids, and not all can be crushed. Ask the pharmacist or GP whether a liquid form or a different formulation is more suitable.
ONS matched to taste and routine
A supplement only helps if it's actually consumed. The best plans rotate flavours and formats, serve them chilled or as part of a routine, and review uptake — an untouched carton does nothing.
Regular reassessment
Swallowing and nutritional needs change. Both the IDDSI level and any ONS should be reviewed periodically by the SLT/dietitian — not left running indefinitely.
How a thickened-fluid or ONS plan usually comes about
The pathway, in plain terms, so you know who does what and where you fit in.
1. A concern is raised
Coughing on drinks, a wet/gurgly voice after eating, repeated chest infections, unplanned weight loss, or poor appetite prompt a referral — often via the GP or ward team.
2. Assessment by the specialist
An SLT assesses the swallow and recommends a safe IDDSI level if needed. A dietitian assesses nutrition (often with 'MUST') and decides whether 'food first', ONS, or both are appropriate.
3. A written plan
The plan names the exact IDDSI level and/or supplement, the amount, and how to prepare and check it. Carers and family are shown how to do it correctly.
4. Monitoring and review
Intake, hydration, weight and comfort are tracked. The plan is adjusted as the person improves or their needs change — supplements aren't meant to run forever.
Honest view: living with thickened fluids
- Lets people with dysphagia keep drinking — and stay hydrated — more safely
- IDDSI gives one shared, internationally recognised language, reducing dangerous mix-ups
- Pre-thickened drinks and ONS take the guesswork out of getting the level right
- Supports independence and quality of life when matched well to the person
- Thickened drinks are less pleasant for many people, so intake — and hydration — can drop
- Getting the level wrong (too thin or too thick) carries real risk; it needs supervision and care
- Powder thickeners must be measured precisely and re-checked, which takes training
- Some medicines interact poorly with thickeners — needs a pharmacy review
Common questions
Can I just thicken drinks at home without an assessment?
No. The correct thickness is specific to the person and is set by a speech and language therapist after assessing their swallow. The wrong level — in either direction — can be dangerous. If you suspect someone has a swallowing problem, ask the GP for a referral; don't start thickening drinks on your own.
What's the difference between the IDDSI levels and the old syrup/custard/pudding terms?
They describe the same idea but IDDSI is the modern UK standard adopted by the British Dietetic Association and Royal College of Speech and Language Therapists. 'Nectar/Syrup' became Level 2 (Mildly thick), 'Honey/Custard' became Level 3 (Moderately thick), and 'Pudding' became Level 4 (Extremely thick). The numbered, colour-coded system reduces confusion between settings.
How do I know a drink is actually at the prescribed level?
IDDSI provides a simple Flow Test: draw the liquid into a standard 10 ml syringe, release for 10 seconds and see how much is left. Levels 1–3 have defined amounts remaining (roughly 1–4 ml, 4–8 ml, and 8 ml or more). Level 4 doesn't flow at all, so the fork-drip or spoon-tilt test is used instead. Your SLT will show you how.
Are oral nutritional supplements the same as ordinary protein or meal-replacement shakes from a shop?
No. Medical ONS are nutritionally formulated for people with disease-related malnutrition or specific needs and are usually recommended through the NHS after screening. Off-the-shelf sports or slimming shakes aren't designed for that purpose and may not be appropriate — especially for someone frail, unwell, or with swallowing difficulties. Always go through a dietitian or GP.
Do I have to use supplements forever?
Usually not. ONS are meant to bridge a gap while addressing the underlying cause and building food intake back up. UK practice is 'food first' wherever possible, with supplements reviewed and stopped once they're no longer needed. Indefinite use without review isn't good practice.
Someone I care for is on thickened fluids but seems to be drinking much less — is that normal?
It's common but it shouldn't be ignored. Thickened drinks are harder work and less appealing, so dehydration is a real risk. Flag a drop in intake to the care team or GP promptly — they can review the level, offer different flavours or pre-thickened options, and check hydration.
Choosing for someone you care for?
Hydration, swallowing and nutrition link up with feeding support, home-care tools and the wider nutrition picture. Explore our plain-English guides — no products sold, no brands pushed, just what to ask and look for.