The danger in tuna is the mercury that the fish absorbs. Once in your body, mercury will either pass through your kidneys, or travel to your brain and supposedly drive you insane. The FDA recommends children and pregnant women do not consume tuna at all. While it's unlikely that eating a massive amount of tuna in one sitting will kill you, it's a good idea to monitor your weekly intake. to visit the Environmental Working Group's tuna calculator to see how much is recommended.