https://www.jogc.com/article/S1701-2163(15)30258-9/pdf

Dear Emmanuel

Following systematic review may be useful in answering your question.

https://www.sciencedirect.com/science/article/pii/S1701216315302589

Have a look here:

Interaction between oral contraceptives and folate :

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5559559/

OCP impair folate metabolism and tend to reduce the serum and erythrocyte levels of folate and increase the urinary FIGLU excretion. The effect is very mild and by itself should not cause a clinically significant folate deficiency unless there are associated contributory factors.

You can benefit from the following links.

1. A.Majid Shojania, Gloria J. Hornady, R.T., Philip H. Barnes. The effect of oral contraceptives on folate metabolism. ,1971,111( 6), 782–791

2. Mahvash Shere, Priya Bapat, Cheri Nickel, Bhushan Kapur, Association Between Use of Oral Contraceptives and Folate Status: A Systematic Review and Meta-Analysis. J Obstet Gynaecol Can 2015;37(5):430–438

3. Shojania AM. Oral contraceptives: effect of folate and vitamin B12 metabolism. Canadian Medical Association Journal. 1982;126(3):244-247.

4. Shojania AM, Hornady GJ, Scaletta D.The effect of oral contraceptives on folate metabolism. III. Plasma clearance and urinary folate excretion. Send toJ Lab Clin Med. 1975 Feb;85(2):185-90.

The effect is mild without any clinical manifestation at all