---
title: "Which common medications affect blood test results?"
type: prep
url: "https://www.test-well.com/prep/medications"
factor: Common medications
aliases:
  - medications that affect lab tests
  - statins liver enzymes blood test
  - birth control hormone test
  - metformin b12
reviewed_at: 2026-09-06
review_status: reviewed
date_published: 2026-09-05
---

# Which common medications affect blood test results?

> Several everyday medicines change results in predictable ways: statins can raise CK and liver enzymes; oral contraceptives raise SHBG and lower free testosterone; metformin lowers vitamin B12 over time; long-term acid reducers (PPIs) lower magnesium and B12; steroids raise glucose and white cells and suppress cortisol; and finasteride roughly halves PSA. Don't stop a prescription for a test — tell the interpreting clinician instead.


## Which results does it affect?

| Marker | Direction | How long the effect lasts | What to do | Source |
| --- | --- | --- | --- | --- |
| CK, AST, ALT (statins) | can raise | While taking; muscle symptoms warrant a CK | Continue; report muscle pain to the prescriber | [FDA — Statin label changes (muscle effects, liver enzymes)](https://www.fda.gov/drugs/drug-safety-and-availability/fda-drug-safety-communication-important-safety-label-changes-cholesterol-lowering-statin-drugs) |
| SHBG (oral contraceptives, oral estrogen) | raises | While taking | Free testosterone and SHBG interpreted in that context | [Testing.com (AACC) — Sex hormone binding globulin (SHBG)](https://www.testing.com/tests/sex-hormone-binding-globulin-shbg/) |
| Free testosterone (oral contraceptives) | lowers | While taking | Note the medication on hormone results | [Testing.com (AACC) — Sex hormone binding globulin (SHBG)](https://www.testing.com/tests/sex-hormone-binding-globulin-shbg/) |
| Vitamin B12 (metformin) | lowers | Develops over months to years | Check B12 periodically on metformin | [NIH ODS — Vitamin B12 fact sheet (metformin interaction)](https://ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional/) |
| Magnesium, vitamin B12 (PPIs, long-term) | lowers | After a year or more of use | Check magnesium and B12 on long-term PPIs | [FDA — PPI label: low magnesium and B12 with long-term use](https://www.fda.gov/drugs/drug-safety-and-availability/fda-drug-safety-communication-low-magnesium-levels-can-be-associated-long-term-use-proton-pump) |
| Glucose, white blood cells (corticosteroids) | raises | While taking; glucose within hours | Note steroid use; A1c reflects the average | [Testing.com (AACC) — Comprehensive metabolic panel](https://www.testing.com/tests/comprehensive-metabolic-panel-cmp/) |
| Cortisol (prednisone, inhaled or topical steroids) | lowers | Suppresses the body's own cortisol while taking | Cortisol testing needs prescriber guidance | [Testing.com (AACC) — Cortisol](https://www.testing.com/tests/cortisol/) |
| PSA (finasteride, dutasteride) | lowers | Roughly halves PSA after 6+ months | Clinicians double the PSA for interpretation | [Testing.com (AACC) — PSA](https://www.testing.com/tests/prostate-specific-antigen-psa/) |
| Biotin (supplement) | interferes | 2–3 days | Stop ≥72 hours before | [FDA Safety Communication — biotin interference with lab tests (2019 update)](https://www.fda.gov/medical-devices/safety-communications/update-fda-warns-biotin-may-interfere-lab-tests-fda-safety-communication) |

## Real effect or interference?

Most medication effects on lab results are real, not artifacts: a statin genuinely raises CK in some people, metformin genuinely lowers B12. The result is accurate; it just needs the medication list to interpret. True assay interference — where the machine misreads — is rarer, and biotin is the main everyday example.

The rule is simple: never stop a prescribed medication to 'clean up' a blood test. List everything you take — prescriptions, over-the-counter drugs and supplements — for whoever reads the results, and ask the prescriber about timing (for example, drawing before a levothyroxine dose) rather than skipping doses.

## Common questions

### Should I stop my statin before a liver test?

No. Mildly raised enzymes on a statin are common and expected; your prescriber decides whether anything needs to change.

### Does birth control affect a testosterone test?

Yes — the pill raises SHBG, which lowers free testosterone. The result is real but reflects the medication, so it's interpreted differently.

### Does metformin really lower B12?

Over years, in a meaningful share of people. Periodic B12 checks are recommended for anyone on long-term metformin, especially with tingling or anemia.

## Related

- [Do you need to fast before a blood test — and which tests does eating affect?](https://www.test-well.com/prep/fasting)
- [Does biotin affect blood test results?](https://www.test-well.com/prep/biotin)
- [How does alcohol affect blood test results?](https://www.test-well.com/prep/alcohol)
- [Should you exercise before a blood test?](https://www.test-well.com/prep/exercise)
- [All preparation guides](https://www.test-well.com/prep)
- [Lab visit guide](https://www.test-well.com/at-the-lab)

## Sources

- [FDA — Statin label changes (muscle effects, liver enzymes)](https://www.fda.gov/drugs/drug-safety-and-availability/fda-drug-safety-communication-important-safety-label-changes-cholesterol-lowering-statin-drugs)
- [Testing.com (AACC) — Sex hormone binding globulin (SHBG)](https://www.testing.com/tests/sex-hormone-binding-globulin-shbg/)
- [NIH ODS — Vitamin B12 fact sheet (metformin interaction)](https://ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional/)
- [FDA — PPI label: low magnesium and B12 with long-term use](https://www.fda.gov/drugs/drug-safety-and-availability/fda-drug-safety-communication-low-magnesium-levels-can-be-associated-long-term-use-proton-pump)
- [Testing.com (AACC) — PSA](https://www.testing.com/tests/prostate-specific-antigen-psa/)
- [FDA Safety Communication — biotin interference with lab tests (2019 update)](https://www.fda.gov/medical-devices/safety-communications/update-fda-warns-biotin-may-interfere-lab-tests-fda-safety-communication)

---

Medically reviewed by Youmna DiStefano, MD (Medical Reviewer, TestWell); last reviewed 2026-09-06. Review policy: https://www.test-well.com/editorial-policy

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