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So is arsenic a poison or a drug?

"It's both," says Joshua Hamilton, professor of toxicology and pharmacology at Dartmouth. "It depends: Are you talking to a Borgia, or are you talking to a physician?"

Poisons surround us. It's not just too much of a bad thing like arsenic that can cause trouble, it's too much of nearly anything. Too much vitamin A, hypervitaminosis A, can cause liver damage. Too much vitamin D can dam-age the kidneys. Too much water can result in hyponatremia, a dilution of the blood's salt content, which disrupts brain, heart, and muscle function.

Even oxygen has a sinister side. "Oxygen is the ultimate toxin," says Michael Trush, a toxicologist at Johns Hopkins Bloomberg School of Public Health. Oxygen combines with food to produce energy, but our bodies also produce oxygen radicals—atoms with an extra electron that damage biomolecules, DNA, proteins, and lipids. "We are oxidizing all the time," says Trush. "The bio¬chemical price of breathing is aging." Which is to say, we rust.

As if everyday poisons aren't enough to angst over, there are nature's more exotic hazards. It's a jungle out there. There are 1,200 kinds of poisonous marine organisms, 700 poisonous fish, 400 venomous snakes, 60 ticks, 75 scorpions, 200 spiders, 750 poisons in more than 1,000 plant species, and several birds whose feathers are toxic when touched or ingested.

Given the treachery of the world, why don't more of us die of poisoning? Because our bodies are designed to protect us from both natural and man-made toxins. The first line of defense, skin, is made of keratin—so waterproof, tough, and tightly woven that only the smallest and most fat-soluble molecules can get through. Our senses warn us of noxious substances; if they fail there is vomiting as backup. Finally, there is the liver, which turns fat-soluble poisons into water-soluble wastes that can be flushed out through our kidneys. The balance tilts over to toxicity only when we step over the threshold of dosage.

Mike Gallo, a toxicologist, knows the principle of threshold from the inside out. Literally. Gallo, a hyper-caffeinated personality wrapped in a wiry frame, is an associate director at the Cancer Institute of New Jersey in New Brunswick. In February 2004, at 64, he was diagnosed with non-Hodgkin's lymphoma. Two weeks later he became both toxicologist and patient at the cancer institute. His oncologist put him on a four-month intravenous diet of toxins, also known as chemotherapy, and he began treatment in a clinic four floors down from his office.

The ingredients of his cocktail included cytoxan, adriamycin, vincristine, prednisone, and Retuxan—toxic enough to cause side effects ranging from vomiting, diarrhea, and weight loss, to liver, heart, and bladder damage, to death from overwhelming infection due to a depressed immune system. In addition, as Gallo will cheerfully tell you, "Almost all cancer drugs are carcinogenic in their own right."

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