TORONTO (Canadian Press) -- The antiviral drugs Tamiflu and Relenza are relatively safe for use in pregnant and breastfeeding women, say the authors of review of data that includes previously unpublished evidence.

The analysis, published electronically on Monday by the Canadian Medical Association Journal, suggested Tamiflu is the best bet for pregnant women, but either drug can be used safely by breastfeeding women who come down with influenza.

The review may assuage concerns of women who contract swine flu and are worried about whether or not to use antiviral drugs. Pregnant women are at higher risk of complications than their non-pregnant peers when they catch seasonal flu. Evidence from some previous pandemics suggests they can be hit disproportionately hard by a strain of pandemic influenza.

"During the current pandemic, we shouldn't hesitate to treat those patients at increased risk," said senior author Dr. Shinya Ito, head of the division of clinical pharmacology and toxicology at Toronto's Hospital for Sick Children.

"In terms of risk-benefit ratio I think it's very clear that the benefit is much more significant to use the drug for pregnant women, even in the first trimester."

The authors, from the Motherisk Program at Sick Kids and the Japan Drug Information Institute in Pregnancy, examined the available data on antiviral drug use in pregnant and breastfeeding women.

They admitted the evidence is limited and said it will be important to monitor what happens with antiviral drug usage by pregnant women during the pandemic.

In addition to looking at reports of use filed to the drugmakers after the fact, they found unpublished Japanese data that followed 90 pregnant women who took Tamiflu during the first trimester of their pregnancies. The first trimester is the period when concern is greatest that chemicals or drugs a mother is exposed to or takes could have a detrimental effect on the developing fetus.

Of those women, only one gave birth to a child with a birth defect. The rate of birth defects in the general population is between one to three per cent. Given that the rate among Tamiflu users isn't higher, it suggests the single case occurred by chance, not as a result of exposure to the drug.

In another study, scientists used a human placenta to see if Tamiflu introduced to the placenta would flow to the fetus. Ito said it took a high concentration of Tamiflu to see even small concentrations of drugs coming out the other end.

The authors suggested Tamiflu is probably the better choice of the two flu drugs to give pregnant women infected with this new strain of H1N1 flu, because there is more safety data on its use in pregnancy.

But they said for women who are breastfeeding, either Tamiflu or Relenza could be used because only small amounts are excreted in breast milk.

The paper said neither drug appears to affect the growth or development of fetuses exposed to them in the womb.

The U.S. Centers for Disease Control said last week that it's receiving reports that some pregnant women who catch swine flu are refusing to take antiviral drugs out of fear it might put their fetuses at risk.

Copyright The Canadian Press, 2009