Mothers who use methamphetamines during pregnancy increase the chance of their child developing behavioural problems at a young age, according to the first study looking at the potential lasting effects of the drug on children.
The behavioural differences; anxiety, depression and moodiness; were tracked in 330 US children aged between three and five years.
While the differences weren't huge, lead researcher Linda LaGasse called them "very worrisome".
Methamphetamine, commonly referred to as P in New Zealand, is a stimulant like crack cocaine.
Earlier research showed P babies have similarities to so-called "crack babies" - including being smaller in size and prone to drowsiness and stress.
Results in long-term studies conflict on whether children of cocaine-using mothers have lasting behaviour problems.
Whether problems persist in young children of P users is also unknown. But University of Auckland senior lecturer Dr Trecia Wouldes, who has worked alongside Dr LaGasse on other studies, said behavioural problems early on aren't a good sign of things to come.
"If you've got children who've got early signs of anxiety and depression, even [if it's] not huge, it does suggest that what's happening is it's going to get worse over time," she said.
The results of a New Zealand study of 107 mothers who used P during pregnancy will be published later this year, she said.
"I think we're going to find similar outcomes ... we're finding that New Zealand kids are tracking the same as US children."
Joseph Frascella, who heads a behavioural division at the National Institute on Drug Abuse in the US, said the research is among "ground breaking" studies examining the effects of substance abuse during pregnancy.
In the study, published this week in the online journal, Pediatrics, mothers or other caregivers completed a checklist asking how often children showed many kinds of troublesome behaviour.
At age three, scores for anxiety, depression and moodiness were slightly higher in P-users' children. These differences persisted two years later.
The older children who'd been exposed to the drug also had more aggression and attention problems similar to ADHD - attention deficit hyperactivity disorder.
Mothers were asked about symptoms, but not if their children had ever been diagnosed with behaviour disorders.
More than half of the mothers who'd used P during pregnancy also used it after giving birth. These women were also more likely to use other drugs during and after pregnancy and to be single mothers.
Last month a study directed locally by Dr Wouldes revealed that NZ women drink more, take more drugs and smoke more cigarettes than their counterparts in the United States.
The study was designed to measure the effects of prenatal exposure to P by studying addicts who were pregnant - but results from a control group of non-methamphetamine users found that use of other drugs among pregnant New Zealanders was startlingly higher.
The effects of such drugs - illicit and legal - on unborn children include miscarriages, low birth weights and mental and growth problems.
- with AP