Health professionals must ensure pregnant women taking the heroin substitute methadone avoid any illicit drugs, a Swiss study suggests.
The combination appears to increase the toxins transferred to the foetus.
Researchers found combining methadone with heroin or cocaine made the placenta - which separates the blood of mother and child - much more permeable.
Writing in Reproductive Biology and Endocrinology, they said women should be carefully monitored.
Methadone has been incredibly important for treatment, but the effects when it is used in combination do warrant further research
Antoine Malek
Zurich University Hospital
They warn that the evidence suggests many pregnant women on methadone continue to take other drugs.
Methadone has long been used as a treatment for opiate addiction.
Used consistently in pregnancy drugs such as cocaine and heroin lead to restricted growth, preterm birth as well as miscarriage, but offering methadone tends to result in an increase in birth weight and the pregnancy being carried for longer.
However methadone is not without problems, including withdrawal symptoms in the baby after birth.
Using 24 placentas from healthy mothers who had just delivered, a team at the University of Zurich tested methadone on its own and with a combination of heroin or cocaine.
They found that when used alone, methadone actually reduced the amount of a test chemical which was able to permeate the tissue.
However, when cocaine or heroin was added to the methadone, the amount of the chemical increased.
They did not test heroin or cocaine in isolation.
Important treatment
"We know that these on their own cause different problems," said Antoine Malek, of the Department of Obstetrics at Zurich University Hospital.
"Methadone has been incredibly important for treatment, but the effects when it is used in combination do warrant further research, particularly as we know so many pregnant women on methadone take other drugs.
"Our research highlights the need for very close monitoring of drug dependent pregnant women."
Research published earlier this year from Scotland found that alongside the prescribed doses of methadone, around 80% of women continued to use illicit drugs during their pregnancy.
Patrick O'Brien, a consultant and spokesman for the Royal College of Obstetricians and Gynaecologists, said the laboratory based results would need to be repeated before their significance could be judged.
"But in any event methadone is by far the best option we have - mainly because of the package that comes with it.
"Women who are on it will come to ante-natal sessions, they will be stabilised, monitored and in a far better position to care for their baby afterwards."
