Drug Interaction Report

Tamoxifen and Cyclophosphamide Injection: Interaction Details

A pharmacist-reviewed analysis of the Tamoxifen and Cyclophosphamide Injection interaction: the documented clinical effects, severity rating, onset, mechanism, and management considerations, with cited references throughout.

AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature

Cyclophosphamide Injection

Cytoxan® Injection Frindovyx Neosar® Injection
+

Tamoxifen

Nolvadex® Soltamox Soltamox®
Dr. Brian Staiger, PharmD, BCPS
Medically reviewed by
Dr. Brian Staiger, PharmD, BCPS
Updated Aug 8, 2026
Editorial policy · Data sources
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Interaction severity
Major
Potentially serious — often needs a change or close monitoring.
How we grade severity & evidence

Severity levels

  • Contraindicated: These should generally not be used together.
  • Major: Potentially serious — often needs a change or close monitoring.
  • Moderate: Can be significant — usually manageable with monitoring.
  • Minor: Usually limited clinical impact.

Evidence grades

  • Established: Well documented — supported by controlled studies or strong clinical data.
  • Probable: Good supporting evidence, though not definitively proven.
  • Suspected: Some evidence suggests this interaction, but it is not well established.
  • Possible: Limited or conflicting evidence; the interaction may occur.
  • Theoretical: Predicted from the drugs' pharmacology; not yet confirmed in people.

Ratings come from the documented interaction literature and are reviewed by a pharmacist. They describe the documented risk of the combination, not what will necessarily happen to you — your dose, timing, and health picture all matter.

Of 89 documented Tamoxifen interactions, 81 are rated major — including this one.
Worried about symptoms right now? Contact your pharmacist or prescriber, or call Poison Control at 1-800-222-1222 (US). Call 911 for an emergency.
Onset
delayed
Evidence
probable
Severity
Major

What happens

An increased risk of thromboembolism

Interaction Deep Dive

The addition of a regimen of cyclophosphamide, methotrexate, and fluorouracil to tamoxifen has been shown to increase the risk of thromboembolic events in postmenopausal women being treated for breast cancer1. Controlled studies are needed to further evaluate whether tamoxifen and cyclophosphamide alone may cause an increased risk of thromboembolism. The effect of tamoxifen on cyclophosphamide, a drug that requires mixed function oxidases for activation, is not known 2.

Why it happens (mechanism)

Unknown

Literature reports

1 report — tap to read

a) In a multicenter randomized trial of 705 postmenopausal women undergoing treatment for breast cancer, the incidence of thromboembolism while undergoing therapy with tamoxifen for two years was compared to tamoxifen therapy for two years plus a six-month regimen of intravenous cyclophosphamide, methotrexate, and fluorouracil (CMF). Out of 353 women receiving tamoxifen plus CMF, 48 women (13.6%) experienced one or more thromboembolic events, compared to nine out of 352 women (2.6%) for the tamoxifen-only group. In addition, significantly more serious thromboembolic events (such as deep venous thromboses, pulmonary emboli, arterial events, or fatal events) were seen in the tamoxifen plus CMF group compared to the tamoxifen-only group 1.

Common questions

Can I take Tamoxifen and Cyclophosphamide Injection together?

An increased risk of thromboembolism Always confirm with your pharmacist or prescriber before making any change.

How serious is the Tamoxifen and Cyclophosphamide Injection interaction?

It is rated major. Potentially serious — often needs a change or close monitoring.

How quickly could this interaction happen?

The documented onset is "delayed". Effects tend to build up gradually over days to weeks.

How strong is the evidence for this interaction?

The evidence is graded "probable". Good supporting evidence, though not definitively proven.

From our Q&A

Real reader questions about these medications, each personally answered by our pharmacist:

Questions for your pharmacist

  • Does my dose of Tamoxifen or Cyclophosphamide Injection need adjusting while I take them together?
  • What symptoms should prompt me to call you or my prescriber right away?
  • Does the timing of my doses matter for this combination?
  • Is there a safer alternative to one of these medications for me?

References (2)

  1. Pritchard KI, Paterson AH, Paul NA, et al: Increased thromboembolic complications with concurrent tamoxifen and chemotherapy in a randomized trial of adjuvant therapy for women with breast cancer. J Clin Oncol 1996; 14:2731-2737.
  2. Product Information: Nolvadex(R), tamoxifen citrate. AstraZeneca Pharmaceuticals LP, Wilmington, DE, 2003. DailyMed
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