Written by a parent, not a doctor. Nothing here is medical advice.

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Medicine

Mercaptopurine (6-MP)

Updated September 2026.

Also known as: 6-MP, Purixan, Purinethol

The nightly maintenance pill. Fakes a DNA building block; the dose is tuned to blood counts.

Mercaptopurine (6-MP) is the nightly pill of maintenance. It is a look-alike of the purine building blocks of DNA. Cells take it in and, over several steps, convert it into thioguanine nucleotides that get built into the DNA copy.

The cell’s proofreading system notices something is wrong but cannot fix it, and after repeated failed repairs the cell dies. Because it works slowly and only on cells that are dividing, it suits years of low daily doses. How fast a child’s body processes it varies a lot from person to person (two genes, TPMT and NUDT15, matter most), which is why the dose is tuned to blood counts and sometimes to a genetic test.

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

Everyday analogy

Mercaptopurine is like faulty bricks that stop a building project.

Mercaptopurine is like faulty bricks that stop a building project. An everyday analogy for one part of how this medicine works. A yellow-skinned builder supports an ordinary brick with both hands, lifts it and seats it in an unfinished brick wall. The builder reaches for a teal brick on a workbench, lifts it, brings the other hand underneath, steps closer and seats it beside the ordinary brick. Several teal bricks are already in the wall. Cracks appear in them; the builder notices the damage and walks back. A yellow-skinned inspector approaches, studies the wall, writes a cross on a clipboard, returns the pencil to its holder and raises a hand to stop the work. The inspector steps back beside the builder. A yellow-skinned operator drives a bulldozer in from the right and lowers its blade. The empty building is pushed down; its original bricks remain visible in the settled rubble. Everyone stays outside the demolition area and the final scene holds. Bricks represent DNA building blocks; teal bricks represent faulty building blocks formed from the medicine inside a cell. Building work represents copying DNA so a cell can divide. The inspector and demolition simplify growth stopping and a damaged cell dying; they are not extra medicines or an immune-cell attack. This analogy shows one part of how the medicine works, with the detailed scientific visualization provided separately. REJECTED One building. Many bricks.Faulty bricks put the project at risk.Work stops. The building comes down. Faulty bricksBuild with faulty bricksReject the damaged buildingStop and take it down

1 / 3 Build with faulty bricks

Build with faulty bricks

  • Teal bricks = faulty DNA building blocks formed when mercaptopurine is changed into active forms inside a cell.

  • Bricks = the building blocks used to make DNA, the cell’s instructions.

  • Building work = copying DNA so a cell can divide. Faulty blocks can disrupt copying and stop a cell dividing.

  • The inspector’s decision and demolition = growth stopping and a damaged cell dying, not extra medicines or an immune-cell attack. This shows one part of how mercaptopurine works.

Press play below for a more accurate visualization.

Interactive visual

How a daily mercaptopurine tablet becomes a fake DNA letter inside leukemia cells

Poster for How a daily mercaptopurine tablet becomes a fake DNA letter inside leukemia cells
Read transcript

## 1. From gut to blood Mercaptopurine crosses the gut lining into blood, so the swallowed medicine can travel onward. ## 2. Entry and resemblance A membrane transporter carries mercaptopurine into the cell. Follow the yellow molecule as it crosses. The two-ring drug resembles a natural purine. This similarity lets cell enzymes act on it. ## 3. The cell activates the drug The first activation enzyme catches mercaptopurine in its pocket, so the cell can begin converting it. Several enzymes build active thiopurine nucleotides. The sugar and phosphate groups make a usable DNA building block. The activated yellow building block is ready. This simplified view combines several chemical steps. Other active metabolites also reduce the supply of natural purines. Here we follow the DNA-damage pathway. ## 4. Copying, then repeated repair The blue template guides copying. Pale building blocks join the new strand one at a time. An activated thiopurine building block approaches the copying machinery. Its resemblance lets it enter the growing DNA. The yellow thioguanine residue is now in DNA. Later copying can leave a mismatch opposite it. Repair machinery checks the mismatch. The yellow residue stays in the template while repair targets the opposite strand. Repair removes a short stretch of the opposite strand. The yellow template residue remains in place. The gap is refilled, but the troublesome template remains. Repair can repeat instead of solving the problem. Persistent gaps can lead to DNA breaks during later copying. This damage can trigger cell death. ## 5. Dismantled from inside DNA damage can make mitochondria leak signals, so dormant caspases wake and begin dismantling the cell. ## 6. Small packets, then cleanup The cell shrinks into small sealed packets. A cleanup cell wraps and clears the fragments.

Side effects

The full list, kept up to date:

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