Some new work has been done on the effects to the human body in space. Basically, it's not good. One comment I read is "we're stuck here". Microgravity wreaks havoc with our bodies, but also does radiation (cancers). Apparently, the kidneys are especially vulnerable (kidney stones, renal failure). Given that it would take about 4 years to reach Mars, and the longest human stay in space was just over 400 days, plus there would be very little protection once anyone reached Mars, it isn't practical. People's lives would be significantly shortened, and/or they might need dialysis by the time they returned. If they lived long enough to return.

And that's not even getting into the money side of things. (What's the financial benefit of putting people in space instead of robots? Is some space activity -like mining asteroids- even worth the cost of robots?)

Better try to take care of our world. We're not feasibly going anywhere else anytime soon. Well, not if we expect to live very long.

scientificamerican.com - Why well never live in space.

Nature: Cosmic kidney disease: an integrated pan-omic, physiological and morphological study into spaceflight-induced renal dysfunction