Since you are underweight, I suggest that you eat raw sunflower seeds, almonds and cashews(if you aren't allergic to them). These nuts and seeds have plenty of calories, and in addition they have plenty of protein, but most important is that they are loaded with minerals. Many who are mercury toxic are deficient in minerals, especially magnesium and zinc.

You might also want to consider doing a one month parasite cleanse. Many here have had parasites.

I suggest that you stop the Nexium ASAP. Why was it prescribed, based on symptoms? The symptoms of too little stomach acid are very similar to those of having too much stomach acid. Too little stomach acid can make you very succeptible to candida and parasites. I suggest that you do a Google search on stomach acid deficiency.

Are you eating well, or do you feel nausious and have little appetite?