Health Risks of Meditation?

One hears a lot about the benefits of meditation; everything from calming yourself and lowering your blood pressure to working towards purifying your mind and finding freedom from suffering. Even as a newer meditator, I feel a better sense of physical and emotional well being. But I'm interested to know if experienced meditators and teachers know of a down side to meditation either physically or mentally. Does so much time spent in one position lead to physical problems such as circulation problems or back issues or even mental breakdown during intense periods of meditation? And if so, are there things one can do to avoid such issues that might interfere with continued meditation?