When magnesium is not absorbed from the GI tract it moves on to the large bowel, and is a laxative. It is a sign you are taking too much rather than too little. Or the type (if Oxide for example) is being used. All of the magnesium orals are only partly absorbed. Not near 100% at all. In fact calicum (when not used with Vit D is only absorbed about 20% per dose. People don't know that.

Our bodies make CoQ-10.... so unless something is happening to upset that chemistry, it is not considered an essential nutrient. Magnesium is essential, Vit C essential, and many of the B's essential including B12.

We make carnitine, taurine, CoQ-10, normally, so when these supplements fail to show improvements of any kind, after a few months, it is probably safe to stop using them.

There is no general rule... but after you read about them for a while you get the impression of which are critical and which are less so. It is a learning process.

CoQ-10 does many things...mostly to help muscle energy requirements, but it works in the mitochondria for any energy purpose. In general when I feel "better" in any way on something, I keep it. We are lucky that most of the PN supplements are rather benign and won't hurt to use for a short time. They tend to be expensive overall, but not hurtful. Other disease states do not have this luxury, that we have here.