Dose, dosage – really, what is the difference?  I really didn't know and I write about pharmacy a lot.  This article, 11 tricky word pairs that can fonduse writers by Laura Hale Brockway explains it so nicely – dosage is the amount of med you take over a period of time, e.g., 1x a day for a month, and a dose is the amount you take at one time–10mg in the morning.

There's a lot of other good ones here, too, such as averse and adverse, further and farther, drag and drug.  Some, like seasonal and seasonable, are so close, you can understand why they are misused.