You don't say why you're taking the Nexium, so there's no way to know if it's even going to help with what you're taking it for, so that's a start. I mention this because a lot of docs hand out prescriptions like candy because they know so little about medicine and meds are pretty much how they do their jobs. But since you're taking this only for a short period of time, lets assume it's actually suited to what you want to do. If it's only for two weeks, I wouldn't worry yourself about anything. It's a very short period of time, not long enough to cause severe disruptions. The major problem with any drug that suppresses digestive acid is that you have to have it in order to digest protein and break down some minerals, so the body has no choice but to react by producing even more stomach acid as a result. So with long-term use of any antacid of any type, whether it's Tums and it's unabsorbable calcium or a protease inhibitor, there can be long-term problems. But you're only intending to use it for two weeks, which is how you should use these drugs if it comes to it being necessary to use them, which is short-term. It's not long enough to cause your body to make any major adjustments to compensate, nor for any long-term digestive problems to arise. So again, don't worry about it. Now, if you have a long-term problem, in this area it's best for most people to treat it more slowly with natural remedies that tackle inflammation in that area and treat it in a more systemic and gentle fashion and change your diet or look at any meds you're taking or bad habits picked up that might be causing the problem. As for magnesium absorption, just make sure, first, you're using a good product and that you're actually absorbing it. Some powders, for example, have been left out in the light too long either in processing or by you and that can cause oxidation. Some forms you'll absorb better than others. Only lab tests will tell you if your levels are good. Minerals are best absorbed when taken with a meal, and it doesn't matter which meal, although do watch if certain outcomes happen with magnesium that can show you're taking too much, such as loose stools or fatigue. The only time y0u take magnesium not with a meal is if you're using it to prevent cramps say from medication like antidepressants or to help you sleep, in which case it is often recommended to be taken before bedtime. All the best.