NOW Magnesium Oxide

Our assessment

The evidence shows that magnesium oxide is a common and affordable form of magnesium, but its absorption is lower than other forms. For general bone and nerve support, it can be a useful option, but you may need to take it with food to improve tolerance. Data for the specific ingredient magnesium oxide is limited in the provided data, so we rely on general mineral principles [NIH ODS].

How it works

Think of magnesium oxide as a mineral source that your body uses for bone mineralization and neuromuscular relaxation. It supports bone density and helps regulate nerve signaling and muscle relaxation [NIH ODS]. While your body does not absorb magnesium oxide as efficiently as other forms like citrate, it still contributes to your daily magnesium needs.

Safety profile

FDA received 592,059 total adverse event reports across all supplement categories. For calcium, a related mineral, high levels can cause constipation, nausea, weight loss, extreme tiredness, frequent need to urinate, abnormal heart rhythms, and a high risk of death from heart disease [NIH ODS]. High calcium intakes might also increase the risk of heart disease and prostate cancer [NIH ODS]. Specific side effect report counts for magnesium oxide alone are not available in the provided data.

Our recommendation

Start with a low dose and take it with food to reduce the risk of stomach upset. Be aware that magnesium oxide can interact with medications — calcium supplements may reduce absorption of bisphosphonates, tetracycline antibiotics, fluoroquinolone antibiotics, levothyroxine, and phenytoin [NIH ODS]. Always separate doses by at least 2 hours if you take these medications. For best results, consider a more absorbable form if you experience digestive discomfort [NIH ODS].

Supplement labels contain several key sections regulated by the FDA:

• Serving Size — The recommended amount per dose. Always check this — some products require multiple capsules per serving.
• Amount Per Serving — Shows the quantity of each nutrient in the product, typically in IU (International Units), mg (milligrams), or mcg (micrograms).
• % Daily Value (%DV) — Indicates how much a nutrient in a serving contributes to a daily diet. 5% DV or less is low; 20% DV or more is high.
• Other Ingredients — Lists inactive ingredients like fillers, binders, and capsule materials (e.g., gelatin, cellulose, magnesium stearate).
• Suggested Use — The manufacturer's recommended directions for taking the supplement, including timing and dosage.

Source: FDA Dietary Supplement Labeling Guide