A low or even undetectable serum methotrexate concentration does not exclude severe toxicity after repeated low-dose exposure. The number means something very different here than it does after high-dose methotrexate therapy.

Bottom Line

In repeated low-dose methotrexate toxicity—especially after a weekly prescription is mistakenly taken daily—the serum methotrexate concentration may be low or undetectable despite clinically important mucositis, cytopenias, renal dysfunction, or sepsis. [1] [2]

Do not use a low methotrexate level to rule out repeated-dose toxicity.

Instead, interpret the exposure pattern, symptoms, renal function, and serial blood counts.

Why Can the Serum Level Be Misleading?

Serum methotrexate reflects drug circulating in plasma at the time the sample is drawn.

That is not the same as measuring the biologic injury already occurring in tissues.

Methotrexate distributes intracellularly, and by the time mucositis or marrow suppression becomes apparent, the plasma concentration may already be very low. A cohort of 28 hospitalized patients with low-dose methotrexate toxicity found no meaningful correlation between serum methotrexate concentration and the severity of neutropenia or thrombocytopenia. [1]

The EXTRIP systematic review likewise concluded that therapeutic drug monitoring has little value in low-dose toxicity and noted that significant morbidity has been reported even with undetectable concentrations. [2]

What Should Matter More Than the Number?

First identify the exposure pattern.

A particularly dangerous error is taking a medication intended once weekly every day or more frequently than prescribed. In a 2026 multicenter poison-center study of 54 adults with oral therapeutic dosing errors, 57% developed organ-system dysfunction; mucositis and cytopenias were prominent complications. [3]

Assessment should therefore focus on:

• how many consecutive days methotrexate was taken;

• the cumulative exposure;

• renal function;

• mucositis or oral ulceration;

• fever or systemic illness;

• bruising or bleeding;

• serial CBC trends.

A normal or low methotrexate concentration should not override those findings.

What Changes the Answer?

This principle does not mean methotrexate concentrations are generally useless.

After high-dose methotrexate therapy, timed concentrations are essential for identifying delayed elimination and guiding rescue therapy. The mistake is applying that same concentration-based logic to repeated low-dose therapeutic toxicity.

For the full distinction between single acute ingestion, repeated low-dose errors, high-dose methotrexate delayed elimination, and intrathecal overdose, see Methotrexate Toxicity: Clinical Guideline.

Clinical Pearl

In repeated low-dose methotrexate toxicity, treat the patient and the exposure pattern—not the reassuring serum level.

A low number can coexist with serious mucosal and marrow toxicity.

References

1. Kivity S, Zafrir Y, Loebstein R, Pauzner R, Mouallem M, Mayan H. Clinical characteristics and risk factors for low dose methotrexate toxicity: a cohort of 28 patients. Autoimmunity Reviews. 2014;13(11):1109–1113. doi:10.1016/j.autrev.2014.08.027

2. Ghannoum M, Roberts DM, Goldfarb DS, et al. Extracorporeal Treatment for Methotrexate Poisoning: Systematic Review and Recommendations from the EXTRIP Workgroup. Clinical Journal of the American Society of Nephrology. 2022;17(4):602–622. doi:10.2215/CJN.08030621

3. Moss M, Liss DB, Chambers AJ, et al. Methotrexate toxicity in the setting of oral therapeutic errors: a multicenter retrospective study. Clinical Toxicology. 2026;64(9):801–807. doi:10.1080/15563650.2026.2649907