At least 11 babies have died after their mothers were given Viagra as part of a medical trial in the Netherlands.
In total 93 women were prescribed the drug at 10 hospitals across the country in an attempt to fix growth defects in their unborn children, the Daily Mail reports.
But 17 children developed high blood pressure in the lungs meaning they did not get enough oxygen, and 11 subsequently died.
Between 10 and 15 more women now face an anxious wait to see if their unborn children will be affected. The rest of the babies were not harmed.
The trial has now been cancelled and an investigation is ongoing.
A second trial with the same drug in Canada has also been paused, though there is no indication anyone there has been harmed.
A similar trial was conducted in the UK with results published in December and offered no conclusive evidence of benefits, but also did not suggest any risks.
It was hoped that Viagra, which is commonly prescribed to men to treat erectile dysfunction, would help increase blood flow through the placenta and promote the babies' growth.
Human trials were given the go-ahead after successful experiments on rats.
As part of the trial, 93 pregnant women were given sildenafil, better known as the brand name Viagra, while 90 women took placebo pills.
According to a hospital spokesman, at least 11 babies from the first test group were born prematurely and later died.
In the second test group, where the mothers were given placebos, nine babies died shortly after birth.
The research began in 2015 and was due to run until 2020, with the participation of 350 patients.
The spokesman said: "The Amsterdam UMC has concluded studying the effects of the medication sildenafil, also known as Viagra, which was given to pregnant women whose foetuses showed severe growth defects."
He said: "The likelihood of developing a lung condition was much higher (among Viagra takers), as was the chances of death shortly after birth."
According to doctors, a provisional analysis showed that Viagra "might be detrimental for babies after birth".
The spokesman said: "All those involved are being assisted by the doctors involved in the study. Also, the Health and Youth Care Inspectorate (IGJ) has been made aware."