With such a short history of research in this area, long-term studies are still needed.

Doctors have not yet done long-term studies to know how well this works over time.

One long-term study found that, after 10 years, 20 out of 100 leads had problems.

She also said better procedures were needed for long-term studies.

But there were two more important findings in this long-term study.

Yet there have not been long-term studies to prove safety.

Some critics say the lack of long-term, independent studies is significant.

Long-term studies on its safety and effectiveness are still under way.

There have been no long-term studies, but consequences are already showing up.

To date, there have been several large, long-term scientific studies.