Extended periods in space can lead to all kinds of health problems. The hazard posed by radiation is well known and is the reason NASA and other space agencies impose limits on the amount astronauts are exposed to. There's also microgravity, which has been studied extensively thanks to regular missions to the International Space Station and long-term studies (like the NASA Twins Study). According to this research, extended periods in microgravity will result in muscle atrophy, bone density loss, changes in eyesight, effects on the cardiovascular and endocrine systems, and even genetic changes.