@mollie2 If you're experiencing discomfort after 3 weeks, I strongly recommend telling your Oncologist how you're feeling first thing tomorrow morning. Doctors know that every body is different and can usually offer alternatives or, depending on what your discomfort is, offer treatment options for the symptoms. Diarrhea is common, and Imodium worked for me and others. I was up to 2/day for a while. But that's me. See what your doctor thinks.

All the best to you. Tagrisso has been something of a wonder drug since it was introduced in 2017.