I was on Imuran last fall and losing hair by the handful. It stopped when I started taking folic acid suppliments. The one I chose has 800 mcg of folic acid and 25 mcg of B12.
It doesn't make sense that your doctor upped the dosage as a response to hair loss. It's more likely that he ignored your concern and upped the dosage in spite of it. I know my GI had no interest or sympathy for my concern about the hair loss. His attitude was "Whatever. Take your meds." In the total scheme of my health, the hair loss was a pretty minor issue, so I tried not to let it upset me too much. But that's pretty demoralizing, to wash your hair every morning and feel like half of it's come away in your hands.
I don't know if it can be attributed to any one med, but my hair went from no gray to about 30% gray in the last year. And in the last 3 or 4 months it went from straight with a little bit of wave to spiral curls. It's the weirdest thing. My hair was curly-ish when I was a baby, but nothing like this. I switched from Imuran to methotrexate in July, and I'm wondering if it will go straight again when I go off the methotrexate. (Hopefully sometime this month.) Now that I've gotten used to it, I kind of like the curls, and I'm hoping they stay.