Is there a difference between the red & green 325mg ferrous sulfate pills?

If the label on the red iron pills says that they contain 325 mg as well, they should be the same thing as what you were taking previously. The color difference is usually just a difference between brands. The labels should state all the ingredients and the amount of iron in the supplements. If these match up, you can rest assured that the quality is the same. The FDA must closely regulate that the active ingredients in all brands of OTC products are the same as the name brands, which is why it is safe and effective to take "off brand" medications and they generally do cost much less.

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There could be another reason for your tiredness. Have you added anything to your diet or made any changes that could interfere with absorption of the iron? If you still can't put your finger on anything different, I would suggest going in and having your iron levels tested, which can prove for sure if that is the reason you're more tired and have had a change in stool color. Speak to your doctor and they'll be able to help you get to the bottom of things. All in all, between various brands, these types of supplements are the same and shouldn't affect you negatively. I would just suggest getting it checked out to make sure there isn't another cause for the changes you've noticed. Best wishes!