Walk into any pharmacy and you’ll find shelves stocked with multivitamins, mineral supplements, and fortified tablets promising to fill the gaps in your diet. Yet for certain nutrients, doctors reach for a syringe instead of a bottle. If pills are so convenient, why does an injection sometimes work better?

The answer lies in how your body absorbs, processes, and uses nutrients — and in some cases, swallowing a pill simply isn’t the most effective delivery method.

The Journey a Pill Has to Take

When you swallow a tablet, it doesn’t go straight to work. It has to survive a gauntlet of stomach acid, digestive enzymes, and the lining of your intestines before any of it reaches your bloodstream. This process is called the first-pass effect, and it can dramatically reduce how much of a nutrient actually makes it into circulation.

Some nutrients are fragile and break down quickly in this environment.…