Historically, ships surgeons kept standard weights and balances aboard ship (though they were usually used on dry land at landfalls), but many medications were packaged or bottled in pre-measured dosages. For example, grains of opium were usually kept wrapt in twists of paper: if someone needed laudanum, you'd just bust out the high-proof liquor, pour a measure of it, and then untwist the wrapped of the opium to let the pre-measured grains fall into the rum or whatever and dissolve. As a matter of fact, by the mid-19th-Century, you started seeing the first graduated bottles with teaspoon, dram, and other such markings carved into the bottles themselves: if you need to pour out a dram of so-and-so, you simply poured the liquid out of the bottle until the top of the liquid settled at or slightly below the next lowest marking on the bottle.