The hormones in
HRT are similar to those used in hormonal
contraceptives, and so may be affected by enzyme-inducing drugs, such as
St John's wort (Hypericum perforatum), in the same way as
contraceptives. Reduced effects are therefore possible.
Any interaction would be most likely to be noticed where
HRT is prescribed for menopausal vasomotor
symptoms, but might be difficult to detect where the indication is osteoporosis. Further study is needed to confirm the importance of this possible interaction.