Not all supplement forms are interchangeable. The form determines absorption rate, GI tolerability, and what the supplement actually does in your body.
Magnesium glycinate offers excellent absorption and is ideal for sleep and anxiety. Magnesium citrate is good for general use and constipation. Magnesium oxide has low absorption and is mainly useful as a laxative or antacid.
Calcium carbonate is the most common and affordable form but requires stomach acid to absorb. Calcium citrate works without stomach acid and is the better choice for people on proton pump inhibitors or over age 65.
Vitamin D3 (cholecalciferol) is the preferred form. Research shows it raises blood levels more effectively and maintains them longer than D2 (ergocalciferol). D2 is used in plant-based supplements and prescription high-dose courses.
Cyanocobalamin is the most studied and affordable form of B12. Methylcobalamin is the active form preferred for people with MTHFR gene variants or absorption issues. Sublingual forms of either bypass the need for stomach intrinsic factor.
Ferrous sulfate is the most evidence-backed iron supplement. Ferrous bisglycinate is gentler on the stomach with comparable absorption. Iron should only be supplemented after confirming low levels by blood test.