I've recently had 1000mg of b12 (apparently correct dose for my weight). could this have an effect on my menstrual cycle? my last period was june 3. i have taken a pregnancy test that has come back negative.

Probably not: Several of the B vitamins including B12 are necessary for a normal menstrual cycle, but taking B12 is unlikely to be the cause of missing a menstrual cycle completely. I assume there is a specific reason for taking the B12, because a normal diet would supply enough unless you have a condition in which your body cannot absorb the B12 or your body is unable to effectively use the B12 it gets.

No : B 12 injections do not affect the menstrual cycle . Please repeat the pregnancy test and see your GP .

Vitamin B12: injection will not affect your periods. However if you are exercising and losing weight the the weight loss and intense exercise can cause you not to have periods