Not every baby takes to cow's milk infant formula, which contains a protein that can trigger an allergy.
Symptoms include a rash, swelling around the mouth or eyes, difficulty in breathing and vomiting.
These can show up within minutes or an hour of consuming the milk, said Associate Professor Marion Aw, a senior consultant at the division of paediatric gastroenterology, hepatology and nutrition at National University Hospital.
Some babies may have delayed allergic reactions, where symptoms appear only 24 hours later.
Babies allergic to cow's milk would have blood specks in their stool, she said. "Exposure to cow's milk protein can be in the breast milk." The cow's milk protein can pass through the mum's breast milk to the infant. Or it can come from drinking infant formula, she added.
But not all reactions to cow's milk are caused by an allergy. Some babies may just be lactose-intolerant.
A cow's milk protein allergy arises from an immunological reaction to the protein, while lactose intolerance involves a relative inability to digest lactose, a sugar.
Lactose intolerance usually causes bloating and stools may be on the loose side, said Prof Aw.
While the symptoms of an allergy and an intolerance are different, the two sometimes get mixed up as the symptoms are subtle.
Formula-fed infants who are lactose-intolerant can have milk that list sucrose, a type of sugar, as an ingredient. Other sugars they can take, she said, are glucose, fructose and maltose.
Breastfeeding mothers should exclude cow's milk products from their diet if their babies are allergic to cow's milk protein.
What's in the stores
Go to any supermarket and one will be spoilt for choice - or be confused - by the range of infant formula. Here is a guide on the different types.
COW'S MILK FORMULAS
Most infant formulas are made from cow's milk which has been processed to make it suitable for infants. The American Academy of Paediatrics said these products are treated by heating and other methods to make the milk protein more digestible. More milk sugar, known as lactose, is added to make the concentration equal to that of breast milk, it said.
Lactose is the primary carbohydrate found in human milk, as well as the main source of carbohydrate in most infant milks.
The butterfat is removed and replaced with vegetable oils and other fats that babies can digest more easily. Iron is also added.
GOAT'S MILK FORMULAS
If your baby is allergic to regular formulas based on cow's milk, goat's milk formula will also be unsuitable. The protein content is very similar.
These are made for infants with health disorders or diseases. There are also formulas made specifically for premature babies.
These are for babies who are at risk of or have a proven intolerance to cow's milk protein. They should be used under medical or dietetic supervision, said Unicef UK Baby Friendly Initiative.
These contain high levels of phyto- estrogen, which may have nega- tive effects on babies. They should be fed to infants only under exceptional circumstances and on a doctor's advice, said Unicef UK.
STAGE 1 ( OR FIRST MILK)
This whey-based option is the best type of infant formula to give your baby from birth. It is the only breast-milk substitute needed in the first year of life, said Unicef UK.
STAGE 2 (OR FOLLOW-ON MILK)
These are meant for babies who are older than six months.
Dr Yang Linqi, a paediatrician at Thomson Paediatric Centre, said formula milk comes in different stages as companies try to address the changing nutritional needs of a growing child.
Stage 1 and follow-on formulas are relatively similar, she said.
"It is not absolutely necessary to switch to follow-on formulas. It's definitely safe for your baby to continue with Stage 1 formula."
They contain more sugar than animal milk. These products have not been proven to provide extra nutritional benefits for young children, who should be obtaining their nutrients from food.
Correction note: In an earlier version of the story, we said that formula-fed infants need milk that is free of cow's milk protein, such as those that list sucrose, a type of sugar, as an ingredient. This is incorrect. It should be formula-fed infants who are lactose-intolerant can have milk that list sucrose, a type of sugar, as an ingredient. We are sorry for the error.