Do vitamin B12 and methotrexate interact?
There’s no well-known, high-profile “direct” interaction where vitamin B12 and methotrexate must be taken together or avoided in the way that can happen with some antibiotics or heart meds. The bigger clinical connection is indirect: methotrexate can worsen nutrient status in some people, and B12 matters when evaluating anemia or nerve symptoms that can overlap with methotrexate side effects.
Clinicians often check blood counts and nutritional labs (including B12) in patients who develop anemia or other blood-related issues while on methotrexate, because vitamin deficiency can contribute to the same symptom patterns (fatigue, low blood counts, neuropathy-like complaints).
Why would B12 come up in someone taking methotrexate?
Methotrexate can affect rapidly dividing cells and can contribute to blood-count changes. In some patients, it may also coincide with folate and/or other nutrient issues (depending on dose, duration, diet, and whether folic acid is used). When patients develop anemia, clinicians may look for common causes, and vitamin B12 deficiency is one of the key possibilities because it can cause:
- Macrocytic anemia (enlarged red blood cells)
- Fatigue and weakness
- Nerve symptoms (numbness or tingling)
That’s why B12 is sometimes ordered or supplemented in patients on methotrexate who have signs of deficiency.
Does methotrexate reduce B12 levels?
Methotrexate is most commonly linked in practice with folate-related concerns (and many regimens include folic acid). Vitamin B12 specifically is not the classic “methotrexate-drives-B12-deficiency” story, but B12 levels still get evaluated because deficiency can exist due to diet, absorption issues (like pernicious anemia or GI problems), or other medications.
If your concern is a specific lab trend, the right interpretation depends on your B12 level, your blood counts (MCV, hemoglobin), and symptoms.
Should you take B12 with methotrexate?
If you have confirmed or suspected vitamin B12 deficiency, taking B12 is generally aimed at correcting that deficiency. What matters most is:
- Whether you actually need B12 (based on labs and symptoms)
- Whether you’re already taking folic acid (common with methotrexate)
- Your kidney function and overall medication list (because safety depends on the whole regimen)
Because dosing and the form of B12 (oral vs injection) can differ by cause of deficiency, decisions are best made with your prescriber based on lab results.
What symptoms should prompt checking B12 while on methotrexate?
Consider asking your clinician about B12 (and related labs) if you develop:
- New or worsening fatigue with anemia on labs
- Burning, numbness, or tingling in hands/feet
- Glossitis (sore tongue) or other signs of deficiency
- Unexplained changes in blood counts (especially if MCV is high)
Also seek urgent care for methotrexate red flags like mouth sores, fever/infection, shortness of breath, or severe vomiting/diarrhea, since those can indicate toxicity and need immediate attention.
What labs usually get checked?
In practice, evaluation often includes:
- Complete blood count (CBC) with MCV
- Vitamin B12
- Sometimes folate
- Sometimes methylmalonic acid (MMA) and homocysteine if B12 results are borderline
Your clinician uses the pattern of CBC + B12 (and sometimes MMA/homocysteine) to decide whether supplementation is warranted.
If you tell me your situation, I can be more specific
To give a more accurate answer, share:
- Your methotrexate dose and how long you’ve been taking it
- Whether you also take folic acid (and the dose)
- Your most recent CBC values (especially hemoglobin and MCV) and your B12 level (with units)
- Your symptoms (if any) and any other meds you take