Can metanx make you irritable?

It shouldn't, but anything is possible.

Possible side effects of Metanx :
All medicines may cause side effects, but many people have no, or minor, side effects. Check with your doctor if any of these most COMMON side effects persist or become bothersome:
Drowsiness; feeling of swelling of the entire body; mild diarrhea; numbness or tingling.

Seek medical attention right away if any of these SEVERE side effects occur:
Severe allergic reactions (rash; hives; difficulty breathing; tightness in the chest; swelling of the mouth, face, lips, or tongue); dizziness; itching.

This is not a complete list of all side effects that may occur. If you have questions about side effects, contact your health care provider. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088. You may also report side effects at http://www.fda.gov/medwatch.

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For more information see http://www.drugs.com/cdi/metanx.html