It depends, where is the medical scheduled and your nationality.
If you are a citizen of the UK then you will have to do a government medical when you arrive in Qatar, it consists of a blood test and chest x-ray, they are just checking for any major disease. This will all be taken care of in the first month of you being here.
If you have any known conditions then I would suggest getting it looked at before you come and clarify things with the company. Be up front and open, you as there are things you that might be ok on your Class 1 medical that won't fly with the Qatari government (no pun intended). Don't take chances, there are things which would obviously not be a problem in terms of your day to day health which they might be pedantic about over here.