Cancer is a pretty good excuse for almost anything. You lose your keys in the middle of treatment, you forget someone’s name, you wipe an entire afternoon (and a friend’s hospital visit) from your memory—people are pretty understanding. You do have cancer, after all. But while patients, survivors, and families have long been aware of the mental effects of chemotherapy, doctors and scientists have taken longer to acknowledge and legitimize this phenomenon. Although methods, subject populations, and observed effects vary widely, recent studies overwhelmingly find changes in neurological function after chemotherapy, called “chemotherapy-induced cognitive changes” in the literature and “chemobrain” everywhere else. Its impacts vary based on the type of treatment, but it can affect mood, verbal learning, and memory, among other things. Hence the forgetting of names and where you put the keys. But while observing the effects of the cancer experience is relatively straightforward, pinpointing the chemotherapy drugs as the cause is not so simple. Depression and anxiety, not uncommon for people fighting a life-threatening disease, wreak havoc on the brain: they are associated with decreased working memory and slower processing. Stress doesn’t help either: it elevates glucocorticoid levels, causing problems with memory and plasticity in the hippocampus. Even before the emotional turmoil of treatment, people with cancer may have decreased cognitive function from some underlying cause that messes with both the brain and the eventual cancer site. For example, if your DNA repair mechanism is not up to par, mistakes accumulate in neurons (impacting cognitive function) and in the rest of your body (which could lead to tumor formation). The list of confounding factors continues: the effectiveness of the blood-brain barrier of a particular individual, whether or not a patient has undergone menopause, the type of cancer and treatment regimen… the situation is complicated, and so is teasing out a simple answer. With all those caveats, though, a few fairly simple mechanisms look promising. Placing chemotherapy drugs directly on a dish of brain cells makes them very unhappy, causing both cell death and decreased cell division. If the same drug given to a patient can cross the blood-brain barrier, even in low concentrations, it has similar effects. Another proposal focuses on drugs which work by disrupting the DNA of tumors—unfortunately, they also disrupt the DNA of healthy cells, including those in the brain. Some chemotherapy regimens include the use of molecules occurring naturally in the human body, like cytokines and hormones, which throws off internal methods of regulation and can cause neurological changes. Advances in detection and treatment have changed the landscape of cancer, making survival a more realistic possibility and sparking new discussions on the quality of post-chemo life. The approximately 12 million survivors alive today in the U.S. (compared to 3 million in 1971) attest to this success but also show us that we still have a long way to go. The newfound legitimacy of chemobrain within the medical community is raising the bar for cancer treatment—maybe now survivors will remember all the extra years that medical advances have given them.