What is a chemotherapy cycle?

Chemotherapy is often given in cycles. A cycle is a period of treatment followed by a period of rest.

For instance, you might receive chemotherapy every day for one week, followed by three weeks with no chemotherapy. Those four weeks make up one cycle. The rest period gives your body a chance to recover and build new healthy cells before the next round.

The rest is not a pause in the treatment. It is part of the treatment. Chemo drugs go after cells that divide quickly. Cancer cells do that, but so do several kinds of healthy cells. Blood-forming cells in the bone marrow, the lining of the mouth and gut, and hair follicles all divide fast. The drug cannot tell them apart. Healthy tissue repairs itself faster than cancer does, and the rest period is what gives it that head start. Dose again too soon and healthy tissue never catches up.

Bone marrow usually sets the pace. White blood cells drop to their lowest point about 7 to 14 days after a dose. Doctors call that low point the nadir. Counts then climb back, and they usually reach a safe level before the next treatment is due. A three-week cycle exists mostly because that is how long the marrow needs.

Cycles are counted in days, starting with day 1, the first day you get the drug. So "day 1 of cycle 3" points to an exact spot in your treatment. It is worth learning your own numbering, because side effects tend to arrive on predictable days. Mouth sores, low counts, and fatigue often follow the same rhythm each time.

Schedules vary. Many regimens run every 21 days. Some run every 14 days, a pattern called dose-dense, which usually needs a shot that pushes the marrow to make white cells faster. Others are weekly, or three weeks on and one week off. Some pills are taken two weeks on and one week off. A few newer treatments run continuously, with no rest built in at all.

You will almost always have a blood test before each cycle starts. If the white cell count has not recovered, treatment gets pushed back, often by a week. Neutropenia, meaning a low count of the infection-fighting cells called neutrophils, is defined as fewer than 1,500 of them per microliter of blood. When a regimen carries a 20% or higher chance of causing fever during a low count, guidelines suggest adding a growth factor drug such as filgrastim or pegfilgrastim to shorten the vulnerable window.

A delayed cycle feels like a setback but is routine, and one delay rarely changes how well treatment works. Repeated delays are worth a conversation, since they can reduce the total dose you receive over time.

Treatment schedules vary widely depending on the drugs and the type of cancer. Your doctor or nurse will explain your schedule and what to expect during each cycle. Ask two things specifically: which days are your nadir days, and what number to call at night. A temperature of 100.4 to 100.5 F (38 C) or higher during the nadir needs a call right then, not in the morning.