A question I answered on Quora:
Which medical visits should a candidate do and pass in order to become an astronaut?
It's not really a question of tests to ‘take’ and ‘pass.’ You will be asked questions about your medical history in order to assess the risk to the program. First, to see whether there are any conditions exist that would be a hazard to you and other crew members: conditions like diabetes mellitus (which eliminated me), arrhythmias, apnea, exposure to infectious diseases, any mood disorders, etc. etc. Basically, things you don't want to have a problem with while locked in a ‘room,’ or worse yet, while trapped inside a helmet (which has happened, more on that below). Second, training an astronaut is very very expensive, so these kinds of no-go medical conditions have to be caught early: once trained, NASA wants to keep you on the candidate list as long as it can! For these reasons, medical clearance will be based not only on the medical records you supply, but to a greater extent by the NASA medical team that evaluates you continually during your career. This medical team is highly specialized to ensure that the risk to you, your fellow crew, and the equipment is minimized.
A word on ‘Astronauts’ vs. ‘Cosmonauts.’ The selection process is similar, but also different in some very important areas, which arose from the types of missions that the two programs ran. The US system was developed for Shuttle missions, while the Soviet/Russian system was developed for Salyut/Mir missions. The fundamental differences were mission length, and mission culture. Shuttle was a 14 day maximum trip, while stays on Salyuts or Mir were considerably longer, and only depended on the Soyuz vehicle cycling. The medical selection profiles were therefore quite different, and those vestiges remain. Astronauts are selected, from among other things, for their ability to focus intensely on their portion of the mission and its timetable, which is controlled from the ground. Cosmonauts are selected, from among other things, for endurance and compatibility with each other, and the ability to self-manage their time. Salyut/Mir were under the absolute command of the lead Cosmonaut, following the naval tradition. He was there, he knew what the situation was; the ground was not, and did not.
This led to several difficult situations during the Shuttle-Mir program, and not only between astronauts and cosmonauts, but also between Houston and Korolyov Centers. When first assessing system compatibilities before docking, NASA couldn't believe that the Russian ground control did not know the ‘configuration’ of Mir at every moment. Yes, they knew which Soyuz or Progress was docked to which port, but Korolyov Center did not know the position of every switch, or even what was plugged into what. That was the Commander’s job, not theirs. NASA was stunned at the autonomy.
Long stays by Astronauts on Mir often caused friction between the crew, because each agency exerted absolute control over crew selection (using the above described criteria). This resulted in several disputes, and refusals to cooperate by both crews. One astronaut retreated to their cabin for days at a time, and simply would not interact. On one EVA, an astronaut froze with vertigo (this is actually quite common), and the cosmonauts inside did not respond as he thought they should. The astronaut was expecting to be ‘talked down,’ while the cosmonauts were confident he would talk himself out of the situation.
It will be interesting to watch NASA shift to a longer mission mentality as the objectives change from low Earth orbit to deep space, multi month timelines. NASA might learn a lot from the Russians. Then again, they might not. The commanders were notoriously bad at keeping up with medical records protocols, and Salyut/Mir medical results for long duration missions are a mish-mash at best, and only reliable once they were back on the ground and under the supervision of their full medical teams.