A prescription is a standardized set of instructions from a provider to a pharmacist, and every element on it serves a specific purpose. Once you know the layout and a handful of Latin abbreviations, you can read any prescription or pharmacy label with confidence. The same general skill applies to eyeglass prescriptions, though those use a completely different set of terms.
The Four Parts of a Prescription
Every prescription, whether handwritten or electronic, follows the same basic structure broken into four sections.
The superscription sits at the top. It includes the date, your name, address, age, and sometimes your weight. This is also where you’ll find the familiar “Rx” symbol, which derives from the Latin word for “take.” Think of it as the header identifying who the prescription is for and when it was written.
The inscription is the body of the prescription. It names the drug, its strength (like 500 mg), and its form (tablet, capsule, liquid). If a pharmacist needs to mix multiple ingredients, this section lists each one.
The subscription tells the pharmacist what to do. It’s usually a short instruction like “dispense 30 tablets” or “dispense 120 mL.” This controls how much medication you receive.
The signa, abbreviated “Sig,” is the most important part for you as the patient. It contains the directions for how to take the medication: how much, how often, and any special instructions. The pharmacist translates this line onto the label of your medication bottle.
Common Abbreviations and What They Mean
Prescriptions are full of shorthand rooted in Latin. These abbreviations save space but can look like a foreign language if you’ve never seen them. Here are the ones you’ll encounter most often.
For frequency (how often to take it):
- QD (quaque die): once a day
- BID (bis in die): twice a day
- TID (ter in die): three times a day
- QID (quarter in die): four times a day
- PRN (pro re nata): as needed
- QHS (hora somni): at bedtime
For route (how to take it):
- PO (per os): by mouth
- SL: under the tongue
- PR: by rectum
- IM: intramuscular injection
- IV: intravenous (into a vein)
For dosage units:
- mg: milligrams (the most common unit for pills)
- mcg or ug: micrograms (one-thousandth of a milligram, used for very potent drugs)
- mL: milliliters (for liquids)
So a Sig line reading “1 tab PO BID PRN” means: take one tablet by mouth, twice a day, as needed.
Reading Your Pharmacy Label
The label on your medication bottle is the pharmacist’s translation of the original prescription into plain language. Federal guidelines recommend that labels use simplified, patient-friendly wording, explicit directions, and when possible, include the reason the medication was prescribed. Your label should always show the drug name, strength, quantity dispensed, your name, the prescriber’s name, the pharmacy’s contact information, and the date it was filled.
If the directions on your label don’t match what your provider told you verbally, that’s worth a phone call to the pharmacy. Errors are uncommon, but catching a discrepancy early matters. You should also check the number of refills listed. This tells you how many times the pharmacy can refill the prescription before you need a new one from your provider.
Refill Rules for Controlled Substances
If your prescription is for a controlled substance, different rules apply depending on how the drug is classified. Medications in Schedule II, like certain strong pain relievers and stimulants, cannot be refilled at all. You need a new prescription each time.
Medications in Schedules III and IV, which include many sleep aids and moderate pain relievers, can be refilled up to five times. But there’s a hard expiration: no refills are allowed more than six months after the original prescription date. After that window closes, you need a fresh prescription regardless of how many refills remain.
Electronic vs. Handwritten Prescriptions
Most prescriptions today are sent electronically from your provider’s computer directly to the pharmacy. The system transmits the drug name, strength, dosage form, and a national drug code in a single data package that routes through an intermediary network to your chosen pharmacy.
Electronic prescriptions tend to be more complete than handwritten ones. Pharmacists report that required fields are typically filled in on e-prescriptions, whereas paper scripts often arrive with missing information. The tradeoff is that electronic systems force providers to select very specific packaging options (like choosing between a 30-count and 90-count bottle) that they might normally leave to the pharmacist’s judgment. This occasionally creates confusion that requires a follow-up call between the pharmacy and the prescriber’s office.
Patient instructions on e-prescriptions can still arrive in shorthand or pharmacist-oriented language, so the pharmacy translates them into clearer wording for your label. If your label directions seem vague or confusing, ask the pharmacist to clarify.
How to Read an Eyeglass Prescription
An eyeglass prescription looks nothing like a medication prescription, and the abbreviations are entirely different. Your eye doctor will hand you a small form with a grid of numbers, and here’s what each column means.
OD and OS identify your eyes. OD (oculus dexter) is your right eye, and OS (oculus sinister) is your left eye. Some prescriptions use “OU” to indicate both eyes together.
Sphere (SPH) is the main correction number. It tells you how much lens power you need. A negative number (like -2.00) means you’re nearsighted. A positive number (like +1.50) means you’re farsighted. The higher the number in either direction, the stronger your prescription.
Cylinder (CYL) measures astigmatism, which is an uneven curvature of your cornea. If this column is empty, you don’t have astigmatism. If there’s a number, it indicates how much correction is needed for that irregular shape.
Axis is always paired with the cylinder value. Written as a number between 1 and 180, it specifies the angle on your cornea where the astigmatism sits. Without the axis, the cylindrical correction wouldn’t be oriented correctly in the lens.
Some prescriptions also include an Add value, which is extra magnifying power for reading. This is common in bifocal or progressive lens prescriptions and is typically the same for both eyes. A pupillary distance (PD) measurement, the distance in millimeters between your pupils, may appear on the prescription or be measured separately. You’ll need it if you’re ordering glasses online.