Many parents are incapable of giving their children the correct dose of liquid medicines, claim Australian researchers.
Using household spoons to measure them out could mean a potentially dangerous overdose, they say.
Children under five are at the highest risk of accidental overdose.
The study, presented in Lisbon, tested 97 adults and found 61% measured the wrong dose - 17% measured an overdose and 44% did not give enough.
Dr Rebekah Moles, from the University of Sydney, recruited 97 people visiting day-care centres catering for under-fives from the city.
Of these, 53 were mothers, seven were fathers and the rest were day care staff.
The researchers quizzed the adults on a number of scenarios, asking them what they would do next.
For example they told the parent that their youngest child felt hot and irritable, but was still drinking, eating and playing.
Common over-the-counter medicines were made available, together with a selection of spoons and other dosing devices, and the volunteers chose at what point they would use a medicine, and measured out the dose themselves.
Dr Moles said: "Taking all the scenarios together, 61% of the participants would have given an incorrect dose, and only 75% were able to measure accurately what dose they intended to give."
In total, 17% measured out an overdose of the drug, and 44% did not give enough.
"We found that 7% would give a medicine without taking their child's temperature, and 46% would give medicine when the temperature was less than 38 degrees."
In total, only 14% managed the scenario correctly.
Dr Moles said that almost half of the 119,000 calls received by the New South Wales Poisons Information Centre, which handles emergency calls from across Australia, concerned accidental overdose in children, with 15% needing hospitalisation.
The vast majority of the calls about children involved under-fives, she said.
Presenting her findings at the annual conference of the International Pharmaceutical Federation in Lisbon, she said: "We were surprised and concerned to find that some people thought that medicines must be safe because you can buy them without prescription.
"For example, one parent said to us that if Panadol (a paracetamol-based painkiller) is available over-the-counter, administering a double dose couldn't do any harm."
She said: "There is an urgent need to review the use of children's over-the-counter medicines by parents.
"We are following up this research by using mystery shoppers to visit pharmacies and see what advice they are given when presenting similar scenarios."
Neal Patel from the Royal Pharmaceutical Society of Great Britain, said that it was important for parents to give their child the correct dose.
He said: "If parents are unsure about dosing for children they should always consult the packaging information or seek advice from their local pharmacist."
"Medicines are always supplied in child-proof containers and parents can also help prevent accidental overdose by always keeping medicines out of the sight and reach of children"
Margaret Peycke, from the National Pharmacy Association, said: "Whether you are in Australia or the UK, medicines that are available to buy are safe if used correctly but there are some risks if they are not.
"The medicine should be administered carefully using the spoon or measuring device supplied, to ensure the child does not receive more or less than the recommended dose.
"Household spoons should not be used as a substitute as they do not measure amounts accurately unlike ones that come with the medicine."
Research at John Moores University in Liverpool, also to be presented at a conference this week, concluded that it was feasible to produce "mini-tablets", small enough for a younger child to swallow.