I was on Letrozole for 9 months before surgery, chemo and radiation. My only issue was hot flashes. After all treatment was completed I went on Anastrazole and eventually began to experience terrible joint pain including crippling knee pain. I have other issues as well which I don't know where to place the blame as I also get Prolia shots, and I believe some of the issues were permanent damage from chemo. Anyway, I switched back to Letrozole over a year ago along with adding collagen supplements, and my knees feel mostly normal again. If you read other threads here on the aromatase inhibitors, I think you'll find everyone reacts differently to these drugs, and the generic drug brand itself can make a difference depending on the fillers used. On a personal note, I don't think 5 days is long enough. I took a 30 day break (my oncologist wanted 2 weeks but came around), and when asked told me when returning to the drug that full efficacy was achieved in 2 weeks. This made me think perhaps it meant that I'd need at least two weeks off before the drug (and it's effects) was out of my system but that's just a guess.