Using the anti-depressant Prozac at an early age could lead to emotional problems later in life, US scientists have told the journal Science.
The mice studied grew up anxious and depressed after taking Prozac
Researchers also said pregnant women may also be risking the mental health of their unborn child by using Prozac.
A team at Columbia University in New York found young mice given the drug grew up anxious and appeared depressed in tests on their emotional state.
But Prozac's makers said care should be taken when interpreting rodent studies.
The age of the mice were the equivalent of the last three months of pregnancy to children aged eight.
Prozac is the only anti-depressant which doctors can prescribe to under 18s in the UK.
However, doctors have been reluctant to prescribe it to young children, tending to refer children with depression to child psychologists instead.
Pregnant women are advised to use the drug with caution.
Last month the US Food and Drug Administration ordered drug manufacturers to put strong labels on anti-depressants such as Prozac after they found they increased the risk of suicide.
Columbia University Mark Ansorge, who led the research, said: "Increasingly, these drugs are used to treat emotional disorders in children and pregnant women.
"However, the long-term effects of these medications on brain development are largely unknown."
But he added his study showed that using drugs such as Prozac during the early years of life and late pregnancy "may entail unexpected risks for affective function later in life".
During the study, the scientists injected the mice, which were between four and 21 days old, with Prozac.
Nine weeks after their last injection the mice were given tests to reveal their emotional state.
They showed a reduced inclination to explore when put in a maze, took longer to start eating when placed in a novel setting and were slower to try escaping from a part of their cage that delivered mild electric shocks.
All these behaviours are regarded as signs of anxiety and depression, the team said.
A spokesman for Eli Lilly, the makers of Prozac, said: "Great care should always be taken when trying to interpret mice and rodent studies."
And he added the company does not promote the use of Prozac by children.
Prozac is one of a number of anti-depressants known as selective serotonin re-uptake inhibitors (SSRIs), so-called because they block the re-absorption of the chemical messenger serotonin, causing levels in the brain to remain higher for longer.
Serotonin helps to deliver signals from one brain cell to another.
Marjorie Wallace, chief executive of the mental health charity Sane, said the findings "should not cause undue alarm".
"It is notoriously difficult to ascribe mental states to animals, or to draw conclusions from animal experiments.
"Our concern is that without the SSRIs, which can benefit many thousands of people, we may return to older and more damaging drugs."
Dr David Jewell, of the Royal College of GPs, said: "There are clear warnings about giving anti-depressants such as Prozac to pregnant women because of possible toxic effects on the foetus.
"But, as always, GPs have to balance the benefits and risks.
"In the case of a severely depressed pregnant woman the risks of prescribing could be far lower than the risk of doing nothing."