Molybdenum works its way into your diet through plants, which take it up from the soil. It forms a crucial part of a little enzyme called sulfite oxidase. The enzyme breaks down incoming sulfites and turns them into useful food. Take away molybdenum, and the enzyme, and things get nasty. The lowest-level problem you can look forward to is a severe allergic reaction. Continued molybdenum deprivation causes uric acid to build up in the blood, which brings on horribly inflammed and painful joints. At it worst, molybdenum deficiency takes out the nervous system.
Can’t you just avoid sulfites? It’s possible. The FDA made it illegal to add sulfites to raw fruits and vegetables, but pretty much every other food has some sulfites in it. Preserved fruits, restaurant food (especially shellfish and potatoes), and almost anything you find on the shelf in the grocery store. Beer and wine both naturally contain sulfites. Basically, not enough molybdenum in your system makes it so nearly all your food will kill you.
Too much molybdenum will do the same. Again, sulfur is its partner in crime. Extra molybdenum in your system has such a high affinity for sulfur that it will team up with four sulfur atoms to form tetrathiomolybdate. Tetrathiomolybdate will grab hold of any and all nearby copper, making it unusable for the body. You could chew and swallow copper filings, and with too much molybdenum in your system, you’d still die for lack of copper.
There is one positive side to molybdenum. Scientists have turned a danger into a boon, and worked out a way to use molybdenum to help people with a rare condition which makes accumulation of copper deadly.