dose: the amount of substance administered at one time
The patient was given a dose of 500 mg of paracetamol.
dosage: the amount and rate of administration of a substance
The dosage was changed to 500 mg every 4 hours.
dosage form: the way in which a therapeutic agent is administered (eg a tablet, a capsule, a spray); also referred to as the formulation (especially in agriculture)
dosage regimen: the schedule of doses per unit of of time (eg the number and time of doses)