Over the counter ones aren't really strong enough. I second Kirsty you're really better off eating iron rich food (spinach, beets, red meat) and perhaps something rich in vit C (eg. citrus fruit) x
I used spatone and floradix and an iron tablet as well as eating lots of greens and that worked for me! The prescribed tablets didn’t agree with me
I was prescribed some by the maternity unit, you should be given some by them if it's low, but also really up iron rich food as well because that absorbs easier :)