ACMT Toxicology Visual Pearl: Turning Blue

What commercially available product can cause blue-grey discoloration of the skin and conjunctiva with long term use?

1. Benzocaine, 20% oral gel
2. Colloidal silver, 30 ppm liquid solution
3. Ferrous sulfate, 325 mg tablet
4. Methylene blue, 1% oral solution

[Image from Herbert L. Fred, MD and Hendrik A. van Dijk via Wikimedia Commons]

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