Medicine

Cyclophosphamide

Updated September 2026.

Also known as: Cytoxan

Switched on by the liver, then staples the two strands of DNA together so a dividing cell cannot copy them.

Cyclophosphamide arrives in the body inactive. Enzymes in the liver switch it on, and the active form travels in the blood and into cells. It belongs to a family called alkylating agents, the oldest kind of chemotherapy.

Inside a cell it attaches to the DNA and staples the two strands of the ladder together, a cross-link. A cell that tries to open its DNA to copy it cannot separate the strands, and the damage triggers its self-destruct program. Leukemia cells, dividing fast, are affected most. It is given with a lot of fluid, and often a protective drug, because of one of its breakdown products; the side-effect pages linked below explain that.

I made an everyday analogy and a more scientific visualization of cyclophosphamide.

Everyday analogy

Cyclophosphamide: like gluing a zip together

1 / 3 Trace the open jacket

Trace the open jacket This outline is the pattern for a copy.

Teal glue = the effect of cyclophosphamide after the body activates it.
The zip’s two sides = the two strands of DNA, the cell’s instructions.
Opening and copying the jacket = separating and copying DNA. Joining the strands together makes copying harder. This is a simplified analogy.

Trace the open jacket This outline is the pattern for a copy. Cut and sew the copy Fabric becomes a new jacket. The glued zip cannot open fully Teal glue keeps its two sides joined.

Press play below for a more accurate visualization.

Interactive visual

How the liver switches cyclophosphamide on and how it staples leukemia-cell DNA shut

Read transcript

## An inactive starting form (0–10 seconds) Cyclophosphamide travels in the blood as an inactive starting form. It must first reach the liver. The liver changes the medicine into a carrier form that can travel to other cells. ## The liver starts activation (10–28 seconds) Inside a liver cell, cyclophosphamide settles into an activating enzyme. The enzyme adds an oxygen-containing group. This starts activation; the final DNA-damaging form is not made yet. The changed carrier leaves the enzyme and returns to the blood. ## Entry into a leukemia cell (28–46 seconds) The carrier reaches a leukemia cell. Watch it settle into the fatty membrane, then move through to the inside. Inside the cell, the carrier changes into phosphoramide mustard, the form that can cross-link DNA. This active form passes through a nuclear pore to reach DNA in the nucleus. ## Copy first, then cross-link (46–68 seconds) Normally, the DNA strands separate so the copying machinery can read each strand. The active drug approaches DNA. Its two reactive arms can attach to sites on opposite strands. One arm attaches first. Then the other attaches, joining the two DNA strands with a chemical cross-link. These cross-links act like staples: the strands are tied together where they need to come apart. ## When damage overwhelms repair (68–80 seconds) The copying machinery reaches a cross-link. The linked strands cannot separate normally, so copying stalls. When damage overwhelms repair, the cell can activate its self-destruct program. Mitochondria release signals that activate caspases. ## Sealed packets and cleanup (80–92 seconds) The dying cell shrinks and breaks into small sealed packets. A cleanup cell wraps around the packets and swallows them. The leukemia cell is gone.

Side effects

The full list, kept up to date:

Cyclophosphamide — Together by St. Jude · written for families by a children's cancer hospital
Cyclophosphamide injection — MedlinePlus · U.S. National Library of Medicine

Where I read this

FDA prescribing information on DailyMed · section 12.1, mechanism of action
Childhood ALL treatment (PDQ), National Cancer Institute · where each drug fits in treatment