The short answer
PARP inhibitor is report language that needs context. In treatment plans and biomarker-driven therapy discussions, it is a targeted therapy class that blocks PARP, an enzyme involved in DNA repair. This page explains the plain-language meaning, limits, likely next questions, and why your care team must interpret it with the rest of your results.
PARP inhibitor has a specific meaning in treatment plans and biomarker-driven therapy discussions.
The phrase alone is not the whole diagnosis or treatment plan.
The next step depends on the full report, prior tests, symptoms, and your cancer history.
Ask what the finding changes, what remains uncertain, and when you will review the plan.
Choose how you want to understand this
The full explanation.
The name is a mechanism, not a diagnosis
PARP stands for poly (ADP-ribose) polymerase. It is an enzyme that repairs single-strand breaks in DNA, the small nicks that happen in every cell every day.
A PARP inhibitor is a drug that blocks that enzyme. When repair stalls, the nicks turn into double-strand breaks, which are far harder to fix. A healthy cell has a backup system called homologous recombination, and it survives. A cancer cell with a broken BRCA1 or BRCA2 gene has lost that backup, so it dies.
Scientists call that trap synthetic lethality. Neither the drug alone nor the gene fault alone kills the cell. The combination does. That is why the phrase "PARP inhibitor" almost never appears in a report by itself. It appears next to a gene result.
The four drugs behind the phrase
NIH's LiverTox database records each one, with the year it entered the US market and the dose used.
- Olaparib, brand name Lynparza, approved in 2014. Standard dose is 300 mg by mouth twice daily, supplied as 100 mg and 150 mg tablets. It entered the market for advanced ovarian carcinoma with BRCA1 or BRCA2 mutations, and use later expanded to BRCA-mutated pancreatic and prostate cancers. Lower doses are advised with kidney impairment.
- Rucaparib, brand name Rubraca, approved in 2016 for advanced, refractory ovarian carcinoma with harmful BRCA mutations. Dose is 600 mg by mouth twice daily.
- Niraparib, brand name Zejula, approved in 2017 for advanced, refractory ovarian, fallopian tube, or primary peritoneal carcinoma. Dose is 300 mg by mouth once daily.
- Talazoparib, brand name Talzenna, approved in 2018 for selected HER2-negative, BRCA-positive breast cancer. Dose is 1 mg by mouth once daily, supplied as 0.25 mg and 1.0 mg capsules.
All four continue until the disease progresses or side effects become unacceptable. None is a fixed-length course.
Those strengths are the label's reference figures, listed so you can match the name on your bottle to the drug being described. What you swallow each day is whatever your own oncologist has prescribed. Starting amounts get lowered for kidney function, blood counts and side effects, and they differ between centres, so never adjust your tablets to the numbers on this page.
Why the phrase showed up in your record
The term usually enters a chart in one of three ways.
A genomic test result. A pathology report or tumor sequencing report notes a BRCA1 or BRCA2 alteration, and the report's interpretation section flags PARP inhibitors as a class with matching evidence. That is a laboratory comment, not a prescription.
A germline genetics result. An inherited BRCA finding from a blood or saliva test carries the same flag, and it also has implications for relatives.
A treatment plan or tumor board note. Here the term does mean a decision is being made, usually about maintenance treatment after platinum chemotherapy.
If you cannot tell which of the three you are reading, look at who signed the document. A laboratory director signs a report. An oncologist signs a plan.
What the phrase does not settle
Seeing "PARP inhibitor" in a report does not mean you qualify for one, and it does not tell you which one. The choice depends on your cancer type, whether the BRCA change is inherited or only in the tumor, what chemotherapy you already had, how you responded to it, your blood counts, and your kidney function.
It also does not mean the drug is being started now. Many uses are maintenance, meaning the drug begins after another treatment has already shrunk the cancer.
The side effects that drive monitoring
LiverTox gives each drug its own list, and they overlap heavily. For rucaparib and niraparib the common effects are anemia, fatigue, nausea, diarrhea, constipation, indigestion, abdominal pain, loss of appetite, shortness of breath, and low platelets. Olaparib's list drops the last three and adds cough, muscle and joint pain, headache, and rash. Talazoparib's names fatigue, nausea, vomiting, diarrhea, loss of appetite, headache, hair loss, anemia, low white cells, and low platelets.
Two categories deserve names of their own.
Bone marrow effects. Niraparib and talazoparib are each listed with marked bone marrow suppression. All four list myelodysplastic syndrome, a disorder of blood-forming cells, and olaparib specifically lists acute myelogenous leukemia. These are uncommon but serious, and they are the reason blood counts are checked on a schedule rather than only when you feel unwell.
Liver enzymes. The numbers differ sharply by drug. In trials, aminotransferase elevations occurred in 4% of patients on olaparib. ALT rose in 28% on niraparib and in 33% on talazoparib, with 1% exceeding five times the upper limit of normal. Rucaparib produced ALT elevations in 74%, with values above five times normal in 13%. LiverTox notes that niraparib and rucaparib have not been linked to clinically obvious liver injury despite those numbers. Olaparib, by contrast, is listed with acute liver injury that can be fatal.
Other uncommon but serious effects listed include pneumonitis, meaning lung inflammation, and venous thrombosis with clots for olaparib, and cardiovascular events for niraparib. All four carry embryo-fetal toxicity, so contraception planning is part of starting the drug.
Call the team, do not wait
A temperature of 100.4 degrees F, or 38 degrees C, or higher is a medical emergency. That is CDC's own word for fever during chemotherapy, because these drugs lower the white cells that fight infection. Call your care team immediately, day or night. If you cannot reach them quickly, go to an emergency department and tell them straight away that you are on cancer drug treatment. Do not reach for a fever reducer first without asking; it can mask the problem and delay care.
Go to an emergency department, or call 911, for these:
- Sudden chest pain, or breathlessness that comes on quickly. A clot in the lung behaves like this, and so can severe pneumonitis.
- Bleeding you cannot stop, or black or tarry stools. Low platelets can turn ordinary bleeding into a serious one.
Call the team the same day for these:
- A dry cough that keeps getting worse, or breathlessness building over days. These can signal pneumonitis.
- Swelling, warmth, or pain in one calf. That pattern suggests a clot in the leg.
- Bruising without injury, nosebleeds, or gums that bleed when brushing. These point to low platelets.
- Yellowing of the eyes or skin, or dark urine. These point to the liver.
Questions that turn the term into a plan
- Which gene change was found, and was it inherited or only in the tumor?
- Which PARP inhibitor is being considered, and at what dose?
- Is this maintenance after chemotherapy, or treatment on its own?
- How often will my blood counts and liver tests be checked?
- What dose reduction happens if my counts drop, rather than stopping outright?
- Does my result mean my siblings or children should be tested?
For related reading, see BRCA Gene Mutations Explained, Biomarker Testing and Precision Medicine, and Living With a PARP Inhibitor Day to Day.
Sources
Words to know
Tap any term to see what it means.
Common questions
What does PARP inhibitor mean?
In general, PARP inhibitor is a targeted therapy class that blocks PARP, an enzyme involved in DNA repair.
Does it mean cancer?
It is not appropriate for every cancer; BRCA, HRD, cancer type, prior treatment, and approval context matter.
What should I ask next?
A practical next question is to ask what biomarker supports it, whether inherited genetic testing is needed, and what side effects are monitored.
Questions to ask your doctor
Being prepared helps you get the most out of your appointments. Save or print these questions.
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Your next step
Look up related pathology, imaging, biomarker, and treatment-response language.
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Sources last checked: 2026-08-13 what this meansLast updated: 2026-08-19Next planned review: 2027-07-20
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How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes. We do not employ clinicians and do not intend to — our work is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Editorial review complete — This page completed Cancer Explained's editorial checks (sources, safety, plain language, duplication). It has not been reviewed by a physician or other healthcare professional.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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