Thank you, I appreciate that.
I do take magnesium and a number of other supplements, but have for a while due to my health issues.
Re injection sites, I believe it has something to do with the lipid concentrations and also blood supply in each area.
As was explained to my daughter, subcutaneous injections are deliberately designed not to be immediately absorbed by the body. They sit in the adipose tissue and gradually become absorbed through both capillaries and the lymph system, and this is what makes them last up to a week. Because the biological make up of every injection site is different, it's also one of the reasons some weeks the effect of the injection may not seem to last as long.
Also, things like heat on the injection site, or a hot bath for example, can considerably speed up the rate the medication is absorbed. Hence it may not be effective for as long as expected, or because it's being absorbed faster, can initiate side effects. All very complicated, but interesting nonetheless.
So I think it's certainly worth keeping a record of which sites are used, which seem to last longest and be most effective etc.