I've heard that Vitamin D supplements are useful for preventing cancer, rheumatoid arthritis and multiple sclerosis which runs in my family. Does anyone know the maximum amount that can be safely taken daily?

D3 is the best form of vitamin D
Here is http://www.healingcancernaturally.com/a site for cancer

The best things for cancer prevention is change your diet,
Eliminate caffeine, greasy/fat foods, junk foods, processed/prepacked foods.
Eat lots of fruits and vegetables and homemade foods.
Balance your PH, take anti oxidents,
And even reversal, C0 Q 10
ADD:
Shark cartiledge
Essaic