Women in Northern Ireland and over 70 countries with restrictions have used one of the main websites, Women on Web, the study has found.
The British Journal of Obstetrics and Gynaecology review of 400 customers found nearly 11percent had needed a surgical procedure after taking the medication, reports BBC.
The website claims that it can help reduce the problems linked with unsafe abortions.
But anti-abortion campaigners called the development of such sites "very worrying indeed".
The research into those who had used 'Women on Web' found that about 8 percent did not end up using the medication they had ordered.
Almost 11 percent went on to need a surgical procedure - either because the drugs had not completed the abortion or because of excessive bleeding.
In the study, almost 200 women answered questions about their experiences - 58 percent said they were just grateful to have been able to have had an abortion in this way, while 31 percent had felt stressed but found the experience acceptable.
'Women on Web' posts the drugs only to countries where abortion is heavily restricted, and to women who declare they are less than nine weeks' pregnant.