It is often the case that side effects go away or are greatly reduced after a couple of months.
I had to split my dose - so I take half of it (7.5mg) on Monday night, and half (7.5mg) on Thursday night. MANY fewer side effects that way, at least for me. You could ask about that possibility.
It could also be that your body was already really depleted of folic acid, so it's hitting you harder. If you're taking supplements for a while, that might help eventually.
Also, are you taking the full dose? My doc had me start at a low dose, then gradually take more and more...I think we started with 10mg/week (total), and built up to 25mg/week. That was a couple of years ago...I have been doing really well so we are trying to reduce it again now (I am down to 15mg/week total and doing well so far).
In the end, some people find that they just CAN'T take the oral form, and they do injections. That is always a possibility.
But yes - two weeks is not long, and it might get a LOT better, if you can tough it out for a while.
Sorry you're having trouble....
