I'm not sure how much calcium is in the Blackmores as I use a different multi but calcium supplements will stop the absorption of iron supplements. Rennies have the same effect as they are chockas with calcium. I tend to space out my pills and potions and have some with breakfast, morning tea, dinner, bedtime etc. Makes it easier if you have a few to take and aren't great at swallowing pills as well as being able to separate things that don't work well together. I take my iron before meals and then make sure a few hours have passed before I take my mag/calcium stuff.