Cats can be addicted to tuna, whether it's packed for cats or for humans. Some tuna now and then probably won't hurt. But a steady diet of tuna prepared for humans can lead to malnutrition because it won't have all the nutrients a cat needs. And, too much tuna can cause mercury poisoning. Remember the saying, "Honest as a cat when the meat's out of reach." Your cat will see an open can of tuna next to the sink as a dinner invitation.