Drug & supplement interaction

Fermented Milk + Nafcillin

Nafcillin brand name: Nallpen.

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

Fermented Milk has a moderate interaction with Nafcillin. Theoretically, antibiotic drugs might decrease the effectiveness of fermented milk.

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

Fermented Milk and Nafcillin

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

Moderate

ANTIBIOTIC DRUGS

Why this applies to Nafcillin

Nafcillin falls under ANTIBIOTIC DRUGS — the group Fermented Milk interacts with.

Theoretically, antibiotic drugs might decrease the effectiveness of fermented milk.
Some fermented milk preparations contain live and active organisms, including lactobacilli and bifidobacteria strains. Simultaneously taking antibiotics might kill a significant number of these organisms. Tell patients to separate administration of antibiotics and fermented milk preparations by at least 2 hours.
Severity Moderate Likelihood Probable Evidence D

Interaction 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..

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

Here's the thing to know: this interaction works in the opposite direction you might expect. Nafcillin won't build up or become more dangerous from fermented milk. Instead, the antibiotic could weaken the fermented milk, not the other way around. Nafcillin kills bacteria, and it can't tell the difference between the bug causing your infection and the friendly lactobacilli or bifidobacteria living in your yogurt or kefir.

So if you're hoping for the gut-health perks of those live cultures, taking them right alongside nafcillin may knock out a good chunk of them. This is theoretical, based on how the drug works, and it's not dangerous. Nafcillin itself stays just as effective. To get the most from both, try spacing them at least 2 hours apart, and check with your pharmacist if you have questions.

Contact Dr. Brian directly if you have additional questions on this interaction
References
  1. Pierce A. The American Pharmaceutical Association Practical Guide to Natural Medicines. New York: The Stonesong Press, 1999:19.
  2. Xiao JZ, Takahashi S, Odamaki T, et al. Antibiotic susceptibility of bifidobacterial strains distributed in the Japanese market. Biosci Biotechnol Biochem. 2010;74(2):336-42. 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

Fermented Milk Also Interacts With Other Medications

Fermented Milk has 473 moderate known drug interactions in our database — the Nafcillin one above is one of them. If you take anything else, check it against the full list below.

Moderate 473 No known interaction 2,357 (incl. Nafcillin)
Taking anything else?

Check Another of Your Medications Against Fermented Milk

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

Everything else you take

See Every Supplement That Interacts With Nafcillin

This page covers Fermented Milk only. If you take other supplements — a multivitamin, a sleep aid, something for joints — the Nafcillin page lists 58 supplements it has a documented interaction with, worst first, each one explained.

The most serious there right now: Asarabacca (major), Siberian Cocklebur (major), Alkanna (moderate), Alpine Ragwort (moderate) and Aristolochia (moderate).

By severity: 2 major · 54 moderate · 2 minor.

From our pharmacist’s overview on the Nafcillin page

“Anyone on nafcillin (Nallpen) is usually fighting a serious staph infection, and that makes this a time to be genuinely careful with supplements. Of the 58 with a known interaction on file, the two rated major, Siberian cocklebur and asarabacca, both stress the kidneys and could compound damage while you are being treated.”

Reviewed by 2 · Jul 18, 2026

Open the Nafcillin interaction page

How much people look this up

Where Fermented Milk and Nafcillin 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 Fermented Milk
Top half

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

29 days ago today
Medication Nafcillin
Steady interest

#2,041 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 Fermented Milk

What is Fermented Milk?

  • Digestive health
  • Supporting gut bacteria (probiotics)
  • Lactose-related digestive discomfort
  • General nutrition (protein, calcium)
  • Immune support
  • Diarrhea prevention

Fermented milk is a food made by culturing milk with helpful bacteria or yeast, and it provides protein, calcium, and probiotics. It is generally safe for most people and may help digestion and gut health, though benefits depend on the product and strains used. People with milk allergy or weakened immune systems should be cautious.

Read the full Fermented Milk monograph & all interactions
Medication

About Nafcillin Injection

What Is Nafcillin Injection Used For?

Brand name: Nallpen

NAFCILLIN Injection is used to treat infections caused by staphylococci — specifically strains that produce an enzyme called penicillinase, which makes them resistant to regular penicillin. Susceptibility testing is performed to confirm the bacteria will respond to nafcillin before or shortly after starting treatment.

Nafcillin may also be started right away when a resistant staph infection is strongly suspected, even before test results are available. It should not be used for infections caused by bacteria that would respond to standard penicillin G, or for infections caused by non-staph organisms.

See full Nafcillin Injection drug information
Keep checking

Other Drugs & Fermented Milk

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

0Major
473Moderate
0Minor
2,356No known interaction
473 Moderate 2,356 No interaction
Moderate

473 interactions

Showing 60 of 473 moderate interactions — see all 473 on the Fermented Milk page.

No interaction 2,356 drugs checked — show the list

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

Showing the 60 most looked-up of 2,356 medications checked with no known interaction — see the full Fermented Milk list.

See the full Fermented Milk interactions page
See every supplement checked against Nafcillin, 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
Fermented Milk monograph › Uses, safety, and every drug interaction in one place Nafcillin supplement interactions › Every supplement checked against Nafcillin, ranked by severity Nafcillin Injection 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 Fermented Milk and Nafcillin Together: Common Questions

Can you take Fermented Milk with Nafcillin?
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 Fermented Milk and Nafcillin 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 Nafcillin and Fermented Milk?

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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.