A prescription is a structured document with specific sections, each containing information a pharmacy technician needs to fill the order accurately. Learning to read one means understanding these sections, decoding the Latin-based abbreviations that make up the directions, and catching details like refill limits and substitution rules that affect how the medication is dispensed.
The Three Core Parts of a Prescription
Every prescription is built around three main sections: the inscription, the subscription, and the signa.
The inscription identifies the medication itself, including the drug name, its strength (such as 500 mg), and the quantity to dispense (such as #30 for 30 tablets). This is the “what” of the prescription.
The subscription contains instructions directed at the pharmacy, not the patient. It tells you how to dispense the medication, whether generic substitution is allowed, how many refills are authorized, and any special labeling requirements.
The signa (often abbreviated “Sig” on the prescription) is the directions for the patient. Whatever appears here gets printed on the medication label. A sig line like “i tab PO BID” translates to “take one tablet by mouth twice a day.” Reading the signa accurately is the single most important skill in prescription interpretation, and it depends on knowing the abbreviation system.
Beyond these three sections, every prescription also carries the prescriber’s information (name, address, phone number, DEA number if applicable), the patient’s name and date of birth, the date written, and the prescriber’s signature.
Decoding Sig Codes and Latin Abbreviations
Sig codes are shorthand rooted in Latin. Prescribers use them to compress dosing instructions into a few characters. Here are the ones you’ll encounter most often:
Route of Administration
- PO: by mouth
- SL: under the tongue (sublingually)
- o.d.: right eye
- o.s.: left eye
- o.u.: both eyes
- a.d.: right ear
- a.s.: left ear
- a.u.: both ears
Frequency
- q.d.: every day
- b.i.d.: twice a day
- t.i.d.: three times a day
- q.i.d.: four times a day
- q.o.d.: every other day
- q.h.s.: every bedtime
- q.a.m.: every morning
- q.p.m.: every evening
- q.w.: every week
- p.r.n.: as needed
Timing and Other Modifiers
- a.c.: before meals
- p.c.: after meals
- h.s.: at bedtime
- stat: immediately, right now
- c.: with
- s.: without
- ad lib.: as much as desired
- u.d.: as directed
- d.c.: discontinue
Dosage Forms
- gtt(s): drop(s)
- sol: solution
- susp: suspension
- ung.: ointment
So when you see “ii gtts o.u. q.i.d.,” you’re reading “instill two drops in both eyes four times a day.” A sig that reads “i tab PO t.i.d. p.c.” means “take one tablet by mouth three times a day after meals.”
Calculating Days Supply
Once you’ve decoded the sig, you need to figure out how long the prescription will last. Insurance companies require an accurate days supply on every claim, and getting it wrong triggers rejections.
The basic formula is simple: divide the total number of doses dispensed by the number of doses taken per day. If a prescription is for 60 tablets taken twice daily, the days supply is 60 divided by 2, which equals 30 days.
Liquids work the same way, but you need to convert volume into doses first. A 150 mL bottle of a suspension with directions to take 5 mL three times daily contains 30 doses (150 divided by 5). Divided by 3 doses per day, that’s a 10-day supply.
Inhalers require you to know how many actuations (puffs) the device contains. A standard inhaler with 200 metered doses, prescribed as 2 puffs every 4 hours, means 12 puffs per day (6 dosing intervals times 2 puffs). Dividing 200 by 12 gives roughly 17 days.
Insulin calculations work in units. If a vial contains 1,000 units and the patient uses 40 units per day, the days supply is 25 days.
Eye drops follow the same logic: total number of drops in the bottle divided by drops used per day. Most pharmacies estimate about 20 drops per mL for standard ophthalmic solutions.
Common Unit Conversions
Prescriptions sometimes mix measurement systems, so you need a few conversions memorized. The most practical ones for daily pharmacy work:
- 5 mL = 1 teaspoon
- 15 mL = 1 tablespoon
- 30 mL = 1 fluid ounce
- 240 mL = 1 cup (8 fluid ounces)
- 480 mL = 1 pint
- 1 grain (gr) = 65 mg
- 1 g = 15.4 grains
The teaspoon conversion comes up constantly. When a prescriber writes “5 mL PO t.i.d.,” the label will often read “take one teaspoonful by mouth three times daily” so the patient can measure at home without a syringe.
DAW Codes and Generic Substitution
The Dispense As Written (DAW) code tells you whether the pharmacy can substitute a generic or must dispense the exact brand written. You’ll enter this code during claims processing, and it directly affects what the patient pays. The codes run from 0 to 9:
- DAW 0: No preference indicated. Generic substitution is fine.
- DAW 1: Substitution not allowed by the prescriber. You must dispense the brand.
- DAW 2: Substitution allowed, but the patient requested the specific product dispensed.
- DAW 3: Substitution allowed; the pharmacist selected the product dispensed.
- DAW 4: Substitution allowed, but the generic isn’t in stock.
- DAW 5: Brand dispensed but priced as a generic.
- DAW 7: Substitution not allowed because brand is mandated by law.
- DAW 8: Substitution allowed, but no generic is available on the market.
- DAW 9: Other.
In practice, DAW 0 and DAW 1 account for the vast majority of prescriptions. When a prescriber checks “brand medically necessary” or writes “DAW” on the prescription, that’s a DAW 1.
Controlled Substance Schedules and Refill Rules
Controlled substances carry extra rules that affect how you process a prescription. The federal schedule determines refill limits:
Schedule II medications (strong opioids, certain stimulants) cannot be refilled at all. Every fill requires a brand-new prescription from the prescriber. There is no federal time limit on when a Schedule II prescription must be filled after it’s written, though many states impose their own deadlines.
Schedule III and IV medications can be refilled up to 5 times within 6 months of the date written. After that, a new prescription is needed regardless of remaining refills.
Schedule V medications can be refilled as many times as the prescriber authorizes.
Non-controlled prescriptions typically follow state law, but most can be refilled up to the number the prescriber indicates, often within one year of the date written.
Verifying a DEA Number
Every prescriber authorized to write controlled substances has a DEA number. As a pharmacy technician, you can check whether a DEA number is structurally valid using a built-in math formula. A DEA number has two letters followed by seven digits. The first letter indicates the prescriber type, and the second letter is the first letter of the prescriber’s last name.
To verify the seven digits:
- Add the first, third, and fifth digits together.
- Add the second, fourth, and sixth digits together, then multiply that sum by 2.
- Add the results of those two steps.
- The last digit of that total should match the seventh digit of the DEA number.
For example, if the digits are 1234563, you’d add 1 + 3 + 5 = 9, then add 2 + 4 + 6 = 12, multiply by 2 to get 24, and add 9 + 24 = 33. The last digit is 3, which matches the seventh digit, so the number checks out. A mismatch means the number is invalid, and the prescription needs to be verified with the prescriber’s office before dispensing.