Translation guide
In Japanese, 'magnesia' is almost always expressed using the loanword マグネシア (maguneshia), derived from English. It is used in scientific, industrial, and medical contexts, just like in English.
マグネシアは耐火材として使われます。
Magnesia is used as a refractory material.
The chemical name 'magnesium oxide'. More formal or scientific.
酸化マグネシウムは制酸剤としても用いられます。
Magnesium oxide is also used as an antacid.
Commonly used for the medicine, often in phrases like マグネシアミルク (milk of magnesia).
便秘にはマグネシアが効きます。
Magnesia works for constipation.
Specifically 'milk of magnesia'.
マグネシアミルクを飲んでください。
Please take some milk of magnesia.