Is how long vitamin B6 takes to work for nausea safe during pregnancy?

What does the evidence say about how long vitamin B6 takes to work for nausea during pregnancy?

1. The published dosing schedule reassessed symptoms on day 2 to 3 and again on day 4, plus or minus a day, before changing the dose, which is the closest thing to a sourced answer on timing. 1
2. In that trial, symptom scores improved across the 14-day period, with the largest changes occurring before day 10, so the full effect builds over more than a few doses. 1
3. ACOG recommends 10 to 25 mg of vitamin B6 three or four times a day for nausea and vomiting in pregnancy, and two randomized placebo-controlled trials found 30 to 75 mg a day significantly decreased nausea. 2
4. The Tolerable Upper Intake Level for adults 19 and older is 100 mg a day, and several reports show sensory neuropathy at doses lower than 500 mg a day, so more is not a shortcut to faster. 3
5. Nausea and vomiting of pregnancy usually begins between 4 and 9 weeks and peaks between 7 and 12 weeks, so a rising symptom curve can hide a real B6 effect during the first days. 4

Is how long vitamin B6 takes to work for nausea safe in each trimester?

• First trimester. This is when almost everyone asks the question. Nausea peaks between 7 and 12 weeks, so a few days of no change can feel like forever. Hold a steady dose for the reassessment window before deciding it is not working.
• Second trimester. Symptoms resolve between 12 and 16 weeks for most people, which means improvement here may be the calendar rather than the capsule. That is worth knowing before you credit or blame the B6.
• Third trimester. Persistent nausea this late is unusual enough that timing is the wrong question. Ask your provider to look for other causes rather than adjusting a B6 dose on your own.