Which of the following is a common side effect of immunotherapy?

Fatigue is indeed a common side effect of immunotherapy. This treatment works by stimulating or enhancing the body's immune system to fight cancer cells, and while it can be effective, it often comes with significant physical and emotional demands. Patients undergoing immunotherapy might experience various levels of fatigue due to the immune response activated by the treatment, the cancer itself, or the stress associated with a cancer diagnosis and treatment.

Other options like constipation, weight gain, and hair loss can occur with some treatments but are not typically associated with immunotherapy. For instance, while chemotherapy may lead to hair loss, immunotherapy primarily works by modifying the immune system and does not usually have this effect. Weight gain is less commonly reported in the context of immunotherapy and is more often associated with certain steroid treatments or other conditions. Therefore, fatigue stands out as a prominent and recognized side effect of immunotherapy.