There are three systems of measurement used with medications and drug dosage calculation:

Metric system

Basic unit of solid measurement is the gram, basic unit of liquid measurement is the liter, and basic unit of length measurement is the meter.
Variations of these measurements will include the prefixes of kilo, centi, deci, milli, micro etc.
Unit of measure follows the number
Arabic numbers are used
Fractions are expressed as decimals
Example notations: 3 g; 0.25 mg; 500 mL

Apothecary system

Basic unit of solid measurement is the grain and basic units of liquid measurement include minim, dram, and fluid ounce.
Unit of measure precedes the number
Roman numerals are used for smaller numbers. Arabic numbers are more commonly used if larger numbers.
Fractions are expressed as numerical fractions. A small "ss" may be used to represent 1/2.
Example notations: gr iiss; drams 3 (NOTE: There are symbols to represent dram and ounce which cannot be reproduced here. Refer to page 9 in the Drug Dosage Lab document to see these symbols.)

Household system

Basic units of solid measurement are pounds and ounces and units of liquid measurement include teaspoon, tablespoon, cups, etc.
Unit of measure follows the number
Arabic numbers are used
Fractions are expressed as numerical fractions
Example notations: 1/2 t; 125 lbs