A lot of people here take aza and have no side effects. A common initial side effect is nausea. This can be reduced or totally avoided by starting to take it very slowly, gradually working up to the prescribed dose. The blood tests are to detect 'hidden ' side effects such as low white blood cells or effects on liver.
It doesn't suit everybody though, but you can't know if it will help or how you will react until you try it. There's a blood test that can indicate if a person is likely to benefit but I haven't heard of anybody in the UK getting it.
It slightly increases photosensitivity so you need to be extra careful if photosensitivity is an issue for you
I never heard of any weight problem associated with aza. Prednisone can cause massive weight gain. If you have been on that you will probably be able to reduce the dose when the aza kicks in. Some anti depressants can cause weight gain and I expect there are other meds that can too. Some medicines cause increased appetite which isn't the same as causing weight gain. If you are on meds like that, you need to watch what you are eating very carefully. Some people put on weight even though they are very careful. The medicines can sometimes cause fluid retention and bloating
Sometimes the weight gain is not so much from eating more, as from very reduced activity.
Even a ten minute gentle walk each day can help but some people can't manage even that.
3 stones in 3 months is impressive, and of course distressing. What does your doctor say?
I wonder if it isn't a sign of something else going on. I suggest you keep a list of everything you eat and cut right back on salt to reduce fluid retention. We get enough salt naturally without adding it to our food. It's found in food items you would never suspect, like cornflakes.
Have you had thyroid tests done?
I hope the aza helps