How long to trial a formula
The answer
No manufacturer publishes a trial length for a formula, and no trial we could find measures one. What the NHS does say is that there is no evidence switching to a different formula does any good or harm, and that if you think a brand disagrees with your baby you should talk to a midwife or health visitor. So the honest answer is not a number of days. It is a short list of things to watch, of which weight gain is the only one that settles anything.
Key takeaways
- There is no published trial length for any HiPP formula, and the packs give none.
- The NHS says there is no evidence that switching formula does any good or harm.
- The two to four weeks that circulates is ESPGHAN's diagnostic elimination diet, run under a doctor with a test at the end.
- Stools, wind and a changed feeding pattern are common on any change and settle nothing on their own.
- Weight gain is the measure that turns a set of ordinary symptoms into a reason to see someone.
Is there a published answer?
No. No HiPP pack gives a trial length or a timescale, and we could find no trial that measures one for any formula.4
What is published is the question behind it, and the NHS answers that directly: there is no evidence that switching to a different formula does any good or harm, and if you think a particular brand disagrees with your baby, talk to a midwife or health visitor, who can help you decide whether to try a different type.1
That reframes the page. The question is not how long to wait before switching. It is whether switching is the thing to do at all.
Where does the two to four weeks come from?
A different question, and one worth separating carefully, because the figure circulates as though it were a trial length.
ESPGHAN recommends a short diagnostic elimination diet of two to four weeks, followed by a challenge test, as the cornerstone of diagnosing a cow's milk allergy.2 That is a supervised procedure with a test at the end, prescribed for a baby under investigation.
It is not two to four weeks of trying a tin to see how it goes, and it is not advice about choosing between ordinary formulas. Borrowing the number for that is the commonest misreading on this subject.
What is actually worth watching?
Weight, and whether feeds are being taken. Those two answer the question. Most of what parents watch instead does not.
ESPGHAN is direct about the rest. Changes in stool characteristics, feeding aversion, or occasional spots of blood in stool are common and in general should not be considered diagnostic of a cow's milk allergy, and green or mucous stools should not be considered as symptoms of it in otherwise presumed healthy infants.2
It says that as a caution against over-diagnosis, which it reports is more common than under-diagnosis, and it names what a wrong diagnosis costs: acute reactions, growth faltering, micronutrient deficiencies, and a worse quality of life for the baby and the people caring for them.2
If you do change, how?
One feed at a time. HiPP's advice for moving between its own milks is to replace one bottle a day, adding a bottle of the new formula each day, and never to mix two powders in the same bottle.3
Two practical notes. The scoops are not interchangeable between packs, so measure each powder with the scoop that came with it. And because every feed is made fresh, a changeover is counted in feeds rather than in days of milk already made up.5
When is the answer a person rather than another tin?
When a baby is not gaining weight. When symptoms are getting worse rather than settling. When a reaction frightens you.
At any of those points the useful next step is a diagnosis, not a different brand, and the NHS puts the same decision in the hands of a midwife or health visitor rather than the shelf.12
Products mentioned
Questions parents ask
How long should I give a new formula?
Nobody publishes a figure, so anyone who gives you one is guessing. A fortnight is a reasonable practical answer if a baby is feeding and gaining, because it is long enough to see past the first few days of any change, but it is our reasoning rather than a finding.
Is switching formula safe?
The NHS says there is no evidence that switching to a different formula does any good or harm, and that a midwife or health visitor can help you decide whether to try a different type.
Where does the two to four weeks come from?
A different question. ESPGHAN recommends a short diagnostic elimination diet of two to four weeks, followed by a challenge test, as the cornerstone of diagnosing a cow's milk allergy. That is a supervised procedure with a test at the end, not a period of trying a tin from the shelf, and quoting it as a trial length for an ordinary formula misreads it.
What should change if a formula suits?
Less than people expect, which is the part that catches people out. A baby who was feeding and gaining before will carry on doing both.
What about stools?
They change on any change of formula, and they settle nothing on their own. ESPGHAN's position paper says changes in stool characteristics, feeding aversion and occasional spots of blood are common in infants and should not in general be considered diagnostic of a cow's milk allergy, and that green or mucous stools are not symptoms of it in an otherwise presumed healthy infant. That is a caution against over-diagnosis.
What is actually worth watching?
Weight, and whether feeds are being taken. Those two answer the question between them; the nappies, which are what most parents watch, mostly do not.
How do I change over?
HiPP's own advice for moving between its milks is to replace one feed a day, adding a bottle of the new formula each day, and never to mix two powders in the same bottle. The switching guide has the schedule, and the scoops are not interchangeable between packs.
Can I go back if it does not suit?
Yes, on an ordinary formula, and the same bottle-by-bottle method applies. If a health professional chose the formula, ask them first.
When should I stop trying tins?
When a baby is not gaining weight, when symptoms are getting worse rather than settling, or when a reaction frightens you. ESPGHAN warns that over-diagnosis of cow's milk allergy is more common than under-diagnosis and that a wrong diagnosis carries growth faltering and micronutrient deficiencies of its own, so the useful next step at that point is a diagnosis rather than another brand.
Does the stage number matter while I am working this out?
No. A first infant formula can be a baby's main drink for the whole first year, so there is no clock running alongside the one you are watching.
Sources
- NHS, Types of formula there's no evidence that switching to a different formula does any good or harm; if you think a particular brand disagrees with your baby, talk to a midwife or health visitor; first infant formula is the only formula a baby needs in the first year fetched 23 September 2026
- Vandenplas Y. et al., 2024, Journal of Pediatric Gastroenterology and Nutrition 78(2):386-413 ESPGHAN position paper on cow's milk allergy: the two to four week diagnostic elimination diet followed by a challenge test; changes in stool, feeding aversion and occasional spots of blood are common and not diagnostic; green or mucous stools are not symptoms in an otherwise presumed healthy infant; over-diagnosis is more common than under-diagnosis, and a wrong diagnosis carries growth faltering and micronutrient deficiencies fetched 23 September 2026
- HiPP Germany, Was muss ich beim Umstellen von Milchnahrungen beachten? HiPP's advice on changing between its milks: replace one feed a day, adding a bottle of the new formula each day, and never mix two powders in the same bottle fetched 23 September 2026
- HiPP pack artwork for HiPP 1 Bio Combiotik 600 g, version 8 (May 2025) the pack: a feeding chart by age, a scoop weight, and no trial length or timescale anywhere on it fetched 23 September 2026
- NHS, Making up baby formula feeds are made fresh for each feed, which is why a changeover is counted in feeds rather than in days of prepared milk fetched 23 September 2026
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