Astronaut Scott Kelly spent 340 days on the ISS. When he first arrived back he had to be carried out of the capsule. Here he is a week later in an interview.
In zero g:
Fluids redistribute. Eye sight deteriorates with the constant pressure. This pressure may also affect cognition, though it's hard to separate the effects of a constant pressure in the brain from the constant stress of being in space.
Bones and muscles atrophy. Exercise and good nutrition helps but does not totally prevent this. There are some knock on effects of bone demineralization that aren't obvious. The lost minerals in the bones can clog up the kidneys. Astronauts with long missions go through a 45 day reconditioning program to help build back their bones and muscles. It has been observed that the bone that is regrown is abnormal. I remember reading this but cannot find the source at the moment.
Vestibular disorientation. Your sense of balance is thrown off.
There are 2 real methods for creating artificial gravity. Linear acceleration and centripetal acceleration. Linear acceleration is impractical. We don't have anything that can accelerate close to 1 g for any meaningful amount of time. Rotational acceleration is possible but has a lot of issues. You need a large radius and a slow spin. Here's an excellent breakdown of Artificial Gravity.
Spending extended periods of time in zero g (microgravity) is undeniably harmful. What is still an open question is what levels of gravity are safe in the long term? Is Mars' 0.38 g safe? If not what ranges are safe? On a planet's surface artificial gravity by rotation would be impractical. Maybe there are ways of augmenting the existing gravity, such as wearing a weighted body suit.