Researchers at Cardiff University want to find out if a substance found in vegetables like cabbage and sprouts could ward off cervical cancer.
The capsule contains large amounts of the important substance
They need 3,000 volunteers for the study on the effects of the chemical- diindolylmethane (DIM) - which is sold as a food supplement.
Cancer Research UK is funding the trial.
Cervical cancer affects 3,000 women every year. In Wales, women over 20 are offered a smear test every three years.
Women in Cardiff who have slightly abnormal smear test results are being asked to take part in the study.
The research is being led by Professor Alison Fiander.
"The study is looking at whether a food supplement called DIM is able to reverse minor or mild cervical smear abnormalities and prevent the development of more serious abnormalities in the future," she explained.
"DIM is formed in the body naturally during the digestion of cruciferous vegetables such as broccoli, sprouts and cabbage.
"In fact you'd need to eat more than two whole raw cabbages a day to obtain the same amount of DIM as is present in the capsules we're using in the study."
If an abnormality is detected in a test, women are checked again after six months. The research team want women to join their study during that six-month gap.
Sue Ashman, a research nurse involved in the trials, has to explain the procedure to women who are considering taking part.
"We just try and give a detailed breakdown of the trial, saying exactly what's involved, that the trial runs for six months and that they'll be expected to take this supplement every day for six months.
"Just during the time that they would normally be waiting for their repeat smear."
Ms Ashman said recruitment to the trial had been a "little bit slow" so far.
"I think cervical screening is not a very popular test among women and it may be difficult for them to come along, although all the trial means is that you have one additional test.
"But if the trial is successful, then this supplement could provide us with a way of treating these minor abnormalities that doesn't involve any more invasive treatment."