I found that these two words are synonyms and share the same meaning: the amount of a medicine. Can I use both of them in the same sentence?
for example:
The highest dosage of drug A increased [...] while the middle dose did not.
They can often be used interchangeably. A 'dose' is a countable unit of it (1 spoonfuls, etc). The uncountable 'dosage' is what the doctor prescribes.
The following extract from MedLinguistic explains the difference:
Despite repeated emphasis upon the distinction between dose and dosage, these two terms continue to baffle us. As we’ve often been told, dose refers to a specified amount of medication taken at one time.
In the preferred use of dosage, however, the term refers to the administering of a specific amount, number, and frequency of doses over a specified period of time. Dosage implies duration: a “dosage regimen” is a treatment plan for administering a drug over a period of time.