I have been on Imbruvica for 23 weeks and have periodically had muscle cramps. Lately the cramps have been more frequent and in more numerous places. I spoke to a nurse at the pharmacy providing my Imbruvica and she said this is one of the possible side effects of Imbruvica. I can deal with the discomfort but it put my mind at ease that the cramping was not caused by another health problem. She said to speak to my Oncologist in regards to this.