The reason he was asked to stop the herbal and vitamin supplements is because the ONCs want to get a clear picture of what the Iressa is doing. Drug/herb interactions can and do happen. If your father does experience any adverse side-effects, the ONCs will want to know how to manage them.

You could try to see if this trial is also taking place in Japan, India, Hong Kong, etc. If it is, then there may be some data about these herb/drug interactions. Check the drug manufacturer's Web site for these countries.

Good luck,
Jay