Before your blood test
Which common medications affect blood test results?
Short answer
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.
Medically reviewed by Youmna DiStefano, MD · Last reviewed September 2026
Which results does it affect?
| Marker | Direction | How long | What to do |
|---|---|---|---|
| CK, AST, ALT (statins) | can raise | While taking; muscle symptoms warrant a CK | Continue; report muscle pain to the prescriber [source] |
| SHBG (oral contraceptives, oral estrogen) | raises | While taking | Free testosterone and SHBG interpreted in that context [source] |
| Free testosterone (oral contraceptives) | lowers | While taking | Note the medication on hormone results [source] |
| Vitamin B12 (metformin) | lowers | Develops over months to years | Check B12 periodically on metformin [source] |
| Magnesium, vitamin B12 (PPIs, long-term) | lowers | After a year or more of use | Check magnesium and B12 on long-term PPIs [source] |
| Glucose, white blood cells (corticosteroids) | raises | While taking; glucose within hours | Note steroid use; A1c reflects the average [source] |
| Cortisol (prednisone, inhaled or topical steroids) | lowers | Suppresses the body's own cortisol while taking | Cortisol testing needs prescriber guidance [source] |
| PSA (finasteride, dutasteride) | lowers | Roughly halves PSA after 6+ months | Clinicians double the PSA for interpretation [source] |
| Biotin (supplement) | interferes | 2–3 days | Stop ≥72 hours before [source] |
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.
Sources
- FDA — Statin label changes (muscle effects, liver enzymes)
- Testing.com (AACC) — Sex hormone binding globulin (SHBG)
- NIH ODS — Vitamin B12 fact sheet (metformin interaction)
- FDA — PPI label: low magnesium and B12 with long-term use
- Testing.com (AACC) — PSA
- FDA Safety Communication — biotin interference with lab tests (2019 update)
Educational content, not a diagnosis. Lab results are one input; a clinician interprets them with your history and exam.