Which medication is not commonly associated with causing constipation?

Lenalidomide is the correct choice as it is not commonly associated with causing constipation. This medication is primarily used in the treatment of multiple myeloma and certain types of lymphoma. While some cancer treatments are known for gastrointestinal side effects, lenalidomide more frequently leads to other adverse effects such as fatigue, anemia, and increased risk of infection rather than causing constipation.

Other medications listed, such as thalidomide, capecitabine, and vincristine, have been associated with gastrointestinal issues, including constipation. Thalidomide can cause constipation due to peripheral neuropathy, which can affect bowel function. Capecitabine, a chemotherapy drug, may also lead to gastrointestinal side effects, including constipation. Vincristine is particularly notorious for causing constipation due to its neurotoxic effects that can impair bowel motility. Thus, lenalidomide stands out as the least associated with this specific side effect.