If you’ve lost fluid to diarrhoea or vomiting, water alone replaces the volume but not the electrolytes. That is where oral rehydration salts (ORS) come in: a powder dissolved in water containing sodium, potassium, chloride, citrate and glucose in very specific proportions. They are not an upgraded version of sugar-and-salt water. They are an authorised medicine, and the ratio is precisely what makes them work.
That said, you don’t always need them. A healthy adult with mild gastroenteritis who is drinking and eating normally doesn’t need to buy anything. The criterion is who you are and how much you are losing, not whether you have diarrhoea.
Why the ratio matters so much
In the small intestine, glucose is actively absorbed and drags sodium and water along with it. This is sodium-glucose cotransport. During diarrhoea the sodium absorption mechanism is destabilised, but the glucose one stays intact. So a solution carrying glucose in the right amount keeps the intestine absorbing water even while it is losing it. That is the pharmacological basis set out in the AEMPS product information.
It is also why improvising goes wrong. Overdo the sugar and the solution becomes too concentrated, pulling water into the gut instead of absorbing it: the diarrhoea gets worse. Go short on sodium and you don’t replace what you’ve lost. The product information is explicit: the sachet must be made up with water only, in the exact volume stated, and no other ingredients such as sugar should be added.
What is actually in them
The WHO and UNICEF revised the classic formula in 2004 and recommended lowering the osmolarity. With the reduced formula, the need for unscheduled intravenous fluids in children fell by 33%, stool volume dropped by around 20% and vomiting by around 30%.
Several authorised formulas coexist in Spain. Here are three references, in mmol per litre of made-up solution:
| Component | Reduced-osmolarity ORS (WHO) | Sueroral Hiposódico | Sueroral Casen |
|---|---|---|---|
| Sodium | 75 | 50 | 90 |
| Potassium | 20 | 20 | 20 |
| Chloride | 65 | 41 | 80 |
| Citrate | 10 | 10 | 10 |
| Glucose | 75 | 111 | 111 |
| Total osmolarity | 245 mOsm/l | — | — |
The sodium difference between them isn’t a branding detail: it reflects different contexts. Higher-sodium formulas were designed with high-volume diarrhoea in mind; low-sodium ones for the cases more typical of countries with good access to healthcare. If you aren’t sure which suits you, that is exactly the kind of question a pharmacy counter settles in two minutes.
How to make them up and take them
What the product information says, in short:
- One sachet per litre of drinking water (or whatever volume your particular pack states: there are half-litre presentations). The water can be boiled first if necessary.
- Don’t boil the solution afterwards, and don’t add anything to it.
- Drink it at room temperature, within 24 hours of making it up. Keep any leftover in the fridge, and throw it away after 24 hours.
- Slowly. Drinking it too fast can trigger vomiting. If that happens, stop for ten minutes and restart with smaller, more frequent amounts.
On how much to drink, the general guidance in the product information for adults is 200 to 400 ml of solution per loose stool, with no established maximum daily dose: thirst regulates the necessary amount fairly well. For children over one year, the reference is around 200 ml per stool, given slowly, in 25-30 ml amounts every 10-15 minutes. For infants and any small child, the paediatrician sets the regimen: that is not a place to improvise.
One thing that still surprises people: you don’t need a bland diet. Research does not support restrictive diets in viral gastroenteritis. When the appetite comes back, normal eating can resume even if the diarrhoea hasn’t fully stopped.
Who should use them and who doesn’t need them
Not everyone with diarrhoea needs ORS. Under the criteria of the US National Institutes of Health (NIDDK), most healthy adults with viral gastroenteritis can replace fluids and electrolytes with water, juice, broth or sports drinks, and salty crackers help too.
ORS become the right choice in these cases:
- Children, for whom an oral rehydration solution is recommended directly.
- Older people.
- People with a weakened immune system.
- Adults with severe diarrhoea or signs of dehydration.
And there are situations where ORS are contraindicated and you shouldn’t take them on your own initiative. The AEMPS product information lists: kidney failure, diabetes, heavy and continuous vomiting, bowel obstruction or perforation, known glucose malabsorption, inability to swallow, premature babies and infants under one month. It is also worth bearing in mind if you follow a low-sodium or low-potassium diet, because a single sachet supplies meaningful amounts of both.
On isotonic sports drinks: they are not medicines and do not have the composition of an ORS. For a healthy adult with a mild episode they can serve as another source of fluid; they are not a substitute for rehydration salts when those are indicated.
When to see a doctor
Seek medical attention, without waiting, if any of these appear:
- Signs of dehydration: dizziness on standing, passing very little urine or very dark urine, dry mouth and lips, sunken eyes, rapid breathing or a fast pulse. Any of these justifies being seen the same day.
- Continuous vomiting that stops you keeping fluids down. If you can’t drink, the oral route is no longer the answer.
- Blood in the stool, or fever. As well as a reason to get checked, this is a reason not to take over-the-counter antidiarrhoeals on your own.
- Diarrhoea that doesn’t settle within a few days, or that gets worse rather than better.
- Dehydration in a baby, a small child or an older person. The threshold for seeking help should be lower here, and with symptoms in a child the sensible move is to call the paediatrician directly.
- Underlying kidney disease, diabetes or immunosuppression: ask before taking ORS, not after.
This information is general and does not replace assessment by a healthcare professional. Sources used: Product information, Sueroral Hiposódico, AEMPS/CIMA (in Spanish); Product information, Sueroral Casen, AEMPS/CIMA (in Spanish); WHO/UNICEF joint statement on the clinical management of acute diarrhoea (2004); Treatment of Viral Gastroenteritis, NIDDK (National Institutes of Health).