D.B.
Supplements are not FDA approved, therefore, they do not have to go through the same testing as prescription and OTC medications. If you do take a supplement, be sure it is pharmaceutical grade. (I am a pharmacist).
Honestly, your prenatal vitamin (I assume you're still taking it) should provide you with anything you need while nursing. It you have particular concerns, I would check with your OB and get their recommendations. Your best bet is to eat a good diet and get what you need from fruits and veggies.
UPDATE:
Although I respect that the other poster has different information for you, I stand by my response. Supplements are not approved by the FDA. There are always new studies which disagree with previous studies. For example, it was believed at one time that Vitamin E supplementation was beneficial; more recent studies showed an increase risk of death.
From Oxyguard's website:
"FDA disclaimer These statements have not been evaluated by the food and drug administration. These products are not meant to diagnose, treat or cure any disease or medical condition. Please consult your doctor before starting ANY exercise or nutritional supplement program or before using these or any product during pregnancy or if you have a serious medical condition."