What Those Weird Letters on Your Prescription Really Mean
You pick up your prescription, peel off the label, and stare at a jumble of strange letters: Rx, b.i.d., o.d., SC. It’s not a code meant to confuse you-it’s shorthand. But if you don’t know what it means, it can be dangerous. These abbreviations and symbols are the language of pharmacy, built over hundreds of years, and they’re still used today-even as the system tries to phase them out. Understanding them isn’t just helpful; it’s a safety skill.
The Origin of Rx and Other Latin Shortcuts
The symbol Rx on your prescription isn’t random. It comes from the Latin word recipe, meaning "take." Pharmacists have used it since the 1500s to signal the start of a prescription. Back then, Latin was the universal language of medicine across Europe. A doctor in Rome could write the same prescription as one in London, and the pharmacist would understand it. Today, that tradition lives on-but with serious risks.
Modern safety experts say this old system is outdated. The Institute for Safe Medication Practices found that 6.8% of all medication errors in U.S. hospitals are tied to confusing abbreviations. Some of the most dangerous ones? U for units (can be mistaken for "4" or "cc"), QD for daily (often read as QID, meaning four times a day), and MS (could mean morphine sulfate or magnesium sulfate). These aren’t typos-they’re life-threatening mix-ups.
Common Abbreviations You’ll See on Your Label
Here’s what the most common ones actually mean, in plain English:
- Rx = Prescription (from Latin recipe)
- b.i.d. = Twice a day (from Latin bis in die)
- t.i.d. = Three times a day (from Latin ter in die)
- q.d. or QD = Daily (but avoid this one-many pharmacies now write "daily" instead)
- q.i.d. = Four times a day
- p.o. = By mouth (from Latin per os)
- SC, SQ, or SubQ = Under the skin (subcutaneous injection)
- o.d. = Right eye (from Latin oculus dexter)
- o.s. = Left eye (from Latin oculus sinister)
- a.d. = Right ear
- a.s. = Left ear
- PRN = As needed
- OTC = Over-the-counter (not part of your prescription, but often noted)
Notice something? Many of these are Latin. That’s the problem. A patient might see o.d. and think "overdose." That’s exactly what happened in over 2,100 reported cases in U.S. hospitals in 2023. The same goes for SC-if misread as SL (sublingual), insulin could be swallowed instead of injected. That’s not a small mistake.
Why Some Abbreviations Are Being Banned
The Joint Commission, a major U.S. healthcare safety group, has a "Do Not Use" list. It includes:
- U for units → Use "units" instead
- IU for international units → Write "international units"
- QD, QOD → Write "daily" or "every other day"
- MS or MSO4 → Write "morphine sulfate"
- Trailing zeros like 1.0 mg → Use 1 mg
- Leading zeros like .5 mg → Use 0.5 mg
These rules exist because of real harm. In Pennsylvania alone, 12 people died between 2018 and 2022 from errors involving "U" for units. A nurse thought "5U" meant "5 units"-but the prescription was written as "5.0U," and the zero got lost. The machine read it as 50 units. That’s a tenfold overdose.
Canada and Australia have moved toward full English-only prescriptions. The UK banned nearly all Latin abbreviations in 2019. Result? A 28.7% drop in dispensing errors. The U.S. is catching up, but slowly.
How Pharmacies Are Fixing the Problem
Most community pharmacies now use electronic systems that auto-correct dangerous abbreviations. When a doctor writes "b.i.d.", the system automatically changes it to "twice daily" on your label. CVS, Walgreens, and Walmart all do this. You won’t see q.d. anymore-you’ll see "take once daily."
But here’s the catch: not every prescriber uses electronic systems. Some still handwrite prescriptions, especially in rural clinics or older practices. That’s where problems creep in. A 2023 survey found that 67.8% of community pharmacists still get handwritten prescriptions with risky abbreviations. Eye and ear abbreviations (o.d., a.d.) are the biggest offenders-misread in nearly 1 in 5 community pharmacy errors.
Pharmacy technicians are trained to catch these. The Pharmacy Technician Certification Board requires mastery of 72 core abbreviations. But even they can’t catch everything if the handwriting is bad or the abbreviation is obscure.
What You Can Do to Stay Safe
You don’t need to memorize every Latin term. But you do need to be an active participant in your care.
- Ask the pharmacist to explain every instruction on the label. If it says "b.i.d.", ask, "Does that mean twice a day?"
- Check the dose. If it says "1.0 mg," ask if it’s supposed to be 1 mg. Trailing zeros are a known hazard.
- Compare the label to your prescription. If your doctor said "take one pill in the morning," but the label says "q.i.d.", speak up.
- Use the pill organizer. If you’re taking multiple meds, write out the schedule in plain English on the organizer.
- Report confusion. If you’re unsure, don’t guess. Call your pharmacy or doctor. Better safe than sorry.
One Reddit user, u/MedSafetyFirst, shared a near-miss: a prescription said "MSO4." They thought it was morphine sulfate-until they checked the chart and realized it was magnesium sulfate. Two completely different drugs. One could cause cardiac arrest. The pharmacy now requires full spelling for anything that’s ambiguous.
The Future: No More Latin on Prescriptions
The writing’s on the wall. The World Health Organization wants all prescriptions to use plain English by 2030. The U.S. Pharmacopeia’s new rules, effective May 2024, require prescribers to use standardized English terms. AI tools like IBM Watson’s MedSafety AI are already converting old abbreviations to plain language with 99.2% accuracy.
By 2027, experts predict 95% of electronic prescriptions in the U.S. will use only plain English. The days of "t.i.d." and "o.d." are numbered. But until then, you’re still the last line of defense.
What’s Next? How to Keep Your Medications Safe
Even as systems improve, human error remains. The best tool you have is your own awareness. Always read your label. Ask questions. Don’t assume. If you’re on multiple medications, keep a written list of everything you take-including dosages and times-and bring it to every appointment. Pharmacies can help you organize this, often for free.
And if you ever feel unsure-trust that feeling. It’s not being difficult. It’s being smart.
What does Rx mean on a prescription?
Rx comes from the Latin word "recipe," which means "take." It’s the symbol that starts a prescription and tells the pharmacist what to dispense. It’s not an abbreviation for "prescription"-it’s a historical symbol that’s been used since the 1500s.
Is b.i.d. the same as twice a day?
Yes, b.i.d. stands for "bis in die," which is Latin for "twice a day." Most modern prescriptions now write this out as "twice daily" to avoid confusion. But if you see b.i.d., it means take the medication two times a day-usually spaced about 12 hours apart.
What’s the danger with using "U" for units?
The letter "U" for units has caused dozens of deaths in the U.S. because it can be mistaken for a "4" or "0," especially in handwriting. "5U" might look like "50" or "54." That’s why the Joint Commission banned it. Always write out "units" to avoid life-threatening errors.
Why do some labels say o.d. and others say right eye?
o.d. is Latin for "oculus dexter," meaning right eye. But because patients often mistake it for "overdose," most pharmacies now write "right eye" on the label. If you see o.d. on an older label, it means the medication is for your right eye. Same goes for o.s. (left eye).
Are pharmacy abbreviations the same in Australia as in the U.S.?
Australia follows similar safety guidelines as the U.S. and the UK, and has been moving toward plain English on labels since 2020. The Australian Therapeutic Goods Administration recommends avoiding Latin abbreviations. You’ll mostly see "daily," "twice daily," and "as needed"-not q.d. or p.r.n.
Can I ask my pharmacist to rewrite my label in plain English?
Absolutely. You have the right to understand your medication instructions. If your label still uses abbreviations like b.i.d. or o.d., ask the pharmacist to explain them and, if possible, request a printed version in plain English. Most pharmacies will do this without hesitation.
What should I do if I find an error on my prescription label?
Don’t take the medication. Call the pharmacy immediately. If they confirm it’s wrong, ask them to contact your doctor for a corrected prescription. If you’ve already taken it and feel unwell, seek medical help right away. Reporting errors helps prevent them from happening to others.
