n. Memory loss and reduced mental functioning following chemotherapy.
Also Seen As
Dr Janette Vardy from the Sydney Cancer Centre, told the Clinical Oncological Society of Australia Annual Scientific Meeting ‘chemofog’ had previously been assumed to be temporary, but for 20 to 30 per cent of patients, it had been shown to be ongoing. "It is only in the last 10 years that we have recognised ‘chemofog’ or ‘chemobrain’ as a condition," Dr Vardy said. "For a long time some oncologists didn’t believe it was real and it was all in the mind.
—“Chemo may leave brain fog,” Science Alert, November 18, 2009
Mild cognitive impairment following chemotherapy is one of the most commonly reported post treatment symptoms by breast cancer survivors. This deterioration in cognitive function, commonly referred to as "chemobrain" or "chemofog," was largely unacknowledged by the medical community until recent years.
—N. Boykoff, et al., “Confronting chemobrain,” Journal of Cancer Survivorship, September 16, 2009
1994 (earliest)
Memory impairment is very evident, she told me, in support groups with people who are all on chemotherapy. "We call it 'chemo fog' or 'chemo brains'," she told me.
—Sharon Batt, Patient No More: The Politics of Breast Cancer, Scarlet Press, November 01, 1994
This term started out its linguistic life as the two-word phrase chemo fog, and it has been waging a steady — and successful — campaign to become a single word ever since the first appearance of chemofog in 2003. Overall (that is, since 1994) about 60 percent of the available citations use chemofog (compared to 28 percent for chemo fog and 12 percent for chemo-fog). However, if you look at just the past two years, chemofog monopolizes the conversation with about 81 percent of the cites (versus 11 percent for chemo fog and 8 percent for chemo-fog).

I should also point out that the phrase chemobrain (or chemo brain) is far more common, and dates to about 1991.