Although the words “dose“ and “dosage“ look very alike and are both related to medication or radiation, they are not synonyms:
A dose is the quantity administered at one time or the total quantity administered.
A dosage is the regulation or frequency of doses.
Therefore, a dose can be considered a one-shot deal, and dosage (= regimen) as the timing or periodicity.
Reputed reference materials, such as Medical English Usage and Abusage by Edith Schwager, will help you use terminology correctly.
Do you know more words that are erroneously considered synonyms?
Let me know in the comments below!