Drug & supplement interaction

Selenium + Cyclophosphamide

Cyclophosphamide brand names: Cytoxan, Neosar.

Moderate Interaction
Medically reviewed by · Updated October 2026
The bottom line Moderate

Selenium has a moderate interaction with Cyclophosphamide. Theoretically, selenium supplementation may reduce the effectiveness of immunosuppressant therapy.

Read the full interaction report
SeverityModerate
LikelihoodPossible
EvidenceD
What do Severity, Likelihood & Evidence mean?
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Interaction report

Selenium and Cyclophosphamide

Here is the interaction we found between these two, with the evidence behind it.

Moderate

IMMUNOSUPPRESSANTS

Why this applies to Cyclophosphamide

Cyclophosphamide falls under IMMUNOSUPPRESSANTS — the group Selenium interacts with.

Theoretically, selenium supplementation may reduce the effectiveness of immunosuppressant therapy.
In vitro research and preliminary clinical evidence suggests that selenium may stimulate the immune system.
Severity Moderate Likelihood Possible Evidence D

Interaction has been documented in animal or in lab research, or the interaction has been documented in humans but is limited to case reports or conflicting clinical research exists.

Dr. Brian Staiger, PharmD, BCPS
Dr. Brian Staiger explains PharmD, Clinical Pharmacist

Here's the idea behind this one. Selenium may give your immune system a little nudge to be more active. Cyclophosphamide (Cytoxan, Neosar) often works in the opposite direction, calming or suppressing the immune system, especially when it's being used for autoimmune conditions or to control certain cancers. So in theory, large selenium doses could work against what the medication is trying to do.

I want to be honest about how soft this evidence is. It comes mostly from lab and animal studies, not solid human trials, so it's more of a caution than a proven problem. The selenium you get from a normal diet is very unlikely to matter. Concentrated supplements are the real question. Since cyclophosphamide is serious therapy, please don't start a selenium supplement without checking with your oncologist or pharmacist first.

Contact Dr. Brian directly if you have additional questions on this interaction
References
  1. Peretz, A., Neve, J., Desmedt, J., Duchateau, J., Dramaix, M., and Famaey, J. P. Lymphocyte response is enhanced by supplementation of elderly subjects with selenium-enriched yeast. Am.J Clin.Nutr. 1991;53(5):1323-1328. PubMed
  2. Kiremidjian-Schumacher, L., Roy, M., Wishe, H. I., Cohen, M. W., and Stotzky, G. Supplementation with selenium and human immune cell functions. II. Effect on cytotoxic lymphocytes and natural killer cells. Biol.Trace Elem.Res. 1994;41(1-2):115-127. PubMed

What These Ratings Mean

Every interaction above is graded three ways — here’s the key to each label.

Severity how clinically significant the interaction is
Major

Clinically significant. Avoid the combination, or use only under close professional supervision.

Moderate

Usually worth avoiding or managing — may need monitoring, a dose change, or timing apart.

Minor

Generally not serious. Be aware of it and mention it to your pharmacist.

Likelihood how strong the evidence is that the interaction actually happens in people
  • LikelyWell-controlled human studies have demonstrated the likely existence of this interaction.
  • ProbableInteraction has not been documented in well-controlled studies, however, the interaction has been demonstrated in some small human studies or in controlled animal studies in conjunction with multiple case reports.
  • PossibleInteraction has been documented in animal or in lab research, or the interaction has been documented in humans but is limited to case reports or conflicting clinical research exists.
  • UnlikelyInteraction has been demonstrated in animal or in lab research but has been shown not to occur in humans.
Evidence the quality of the studies behind the rating (graded A–D)
  • AHigh-quality meta-analysis (quantitative systematic review).
  • BCase-control study.
  • CConsensus.
  • DAnecdotal evidence.

These ratings are for information only. Never start, stop, or otherwise change your therapy before speaking with your provider.

Keep it in perspective

Selenium Also Interacts With Other Medications

Selenium has 287 moderate, 33 minor known drug interactions in our database — the Cyclophosphamide one above is one of them. If you take anything else, check it against the full list below.

Moderate 287 Minor 33 No known interaction 2,510 (incl. Cyclophosphamide)
Taking anything else?

Check Another of Your Medications Against Selenium

Covers the 2,830 medications we’ve checked against Selenium. Pick one to open its full report.

From our Q&A

Questions About Selenium or Cyclophosphamide

These are real questions from readers, answered by the pharmacists at HelloPharmacist — written by a person, not generated from a database. Each one covers Selenium or Cyclophosphamide, not necessarily both, but if one is close to what you were about to ask it will usually go further than a database entry can:

Not quite your question? Ask our pharmacists directly — it is free, there is no account required, and answers usually come back within 24 hours.

Everything else you take

See Every Supplement That Interacts With Cyclophosphamide

This page covers Selenium only. If you take other supplements — a multivitamin, a sleep aid, something for joints — the Cyclophosphamide page lists 220 supplements it has a documented interaction with, worst first, each one explained.

The most serious there right now: Activated Charcoal (major), Beer (major), Grapefruit (major), Siberian Cocklebur (major) and St. John's Wort (major).

By severity: 6 major · 176 moderate · 38 minor.

Open the Cyclophosphamide interaction page

One more smart check

Taking Selenium as Part of a Dietary Supplement Product? Check It for Interactions

We hold 16,942 real supplement products containing Selenium, each one broken down to its full ingredient list — and combination products often carry several ingredients besides Selenium. Your product may carry other ingredients that interact with Cyclophosphamide too, on top of Selenium — and those would not show on this page. Find your product and we’ll check every ingredient on its label:

How much people look this up

Where Selenium and Cyclophosphamide Sit Against Everything Else

Out of every supplement and medication people look up on HelloPharmacist, here is where these two land this week — and which way each has been moving.

Supplement Selenium
Top 1%

#14 of 1,452 supplements looked up here in the last 7 days

29 days ago today
Medication Cyclophosphamide
Steady interest

#1,454 of 2,830 medications looked up here in the last 7 days

29 days ago today

Position among everything looked up on this site, not a measure of how commonly either one is used or how important it is. A supplement can climb this list because of a news story. It counts every request to the page, automated ones included, and nothing here identifies anyone.

Supplement

About Selenium

What is Selenium?

  • Thyroid health support
  • Antioxidant support
  • Immune support
  • General nutrient supplementation
  • Reproductive/fertility support

Selenium is an essential trace mineral your body needs in small amounts for thyroid function, antioxidant defense, and immune health. Most people who eat a varied diet get enough, and supplements are mainly useful for those who are deficient. Because too much selenium can be toxic, stick to recommended amounts and check with your pharmacist or doctor before supplementing.

Read the full Selenium monograph & all interactions
Medication

About Cyclophosphamide

What Is Cyclophosphamide Used For?

Brand names: Cytoxan, Neosar

Cyclophosphamide is used mainly to treat cancers in adults and, for most products, in children. These include malignant lymphomas (including Hodgkin's disease and Burkitt's lymphoma), multiple myeloma, leukemias, mycosis fungoides, neuroblastoma, ovarian adenocarcinoma, retinoblastoma and breast cancer. It is often used with other cancer drugs.

Some products are also approved for children with biopsy-proven minimal change nephrotic syndrome (a kidney condition) who did not respond to steroids or cannot tolerate them. This use appears on the Cyclophosphamide capsule, tablet and some injection powder labels. It is not listed for Frindovyx or for Cyclophosphamide Injection solution products. One Cyclophosphamide Injection concentrate is for adults only, because of its alcohol and propylene glycol content.

See full Cyclophosphamide drug information
Keep checking

Other Drugs & Selenium

Browse every other medication we’ve checked against Selenium — both the ones with a known interaction and the ones without. Select any drug for the full detail.

0Major
287Moderate
33Minor
2,509No known interaction
287 Moderate 33 Minor 2,509 No interaction
Moderate

287 interactions

Showing 60 of 287 moderate interactions — see all 287 on the Selenium page.

Minor

33 interactions

No interaction 2,509 drugs checked — show the list

We also checked Selenium against these medications and found no known interaction in our sources. Open any page to confirm.

Showing the 60 most looked-up of 2,509 medications checked with no known interaction — see the full Selenium list.

See the full Selenium interactions page
See every supplement checked against Cyclophosphamide, ranked by severity
Sources

Sources & How We Checked

Interaction details are evidence-graded and sourced from the Natural Medicines database. The medication overview comes from MedlinePlus.

Content is written and reviewed by licensed HelloPharmacist pharmacists. See our data sources and editorial standards for how this information is built and checked.

Parts of this content are provided by the Therapeutic Research Center, LLC.

DISCLAIMER: Currently this does not check for drug-drug interactions. This is not an all-inclusive comprehensive list of potential interactions and is for informational purposes only. Not all interactions are known or well-reported in the scientific literature, and new interactions are continually being reported. Input is needed from a qualified healthcare provider including a pharmacist before starting any therapy. Application of clinical judgment is necessary.

© 2021 Therapeutic Research Center, LLC

Keep exploring
Selenium monograph › Uses, safety, and every drug interaction in one place Cyclophosphamide supplement interactions › Every supplement checked against Cyclophosphamide, ranked by severity Cyclophosphamide drug information › The full plain-English guide to this medication Supplement interaction checker › Check any supplement or product against your medications Browse medications A–Z › Every drug we've checked against herbs & supplements Ask a pharmacist › Free, personal answer from our licensed team
Pharmacist Counseling Corner

Taking Selenium and Cyclophosphamide Together: Common Questions

Can you take Selenium with Cyclophosphamide?
There is a moderate interaction, so it may be acceptable with monitoring or timing changes — but check with your pharmacist first. Read the full details on this page and confirm with your pharmacist before combining them.
How serious is the Selenium and Cyclophosphamide interaction?
We rate it Moderate. This combination may require monitoring, a dose adjustment, or separating the times you take each one. Consult your pharmacist before using them together.

Written and reviewed by the HelloPharmacist editorial staff. Our editorial policy

Still Have a Question About Cyclophosphamide and Selenium?

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This information is for education only and is not a substitute for professional medical advice. Always check with your pharmacist or doctor before starting, stopping, or combining supplements and medications.