On a prescription, BID means twice a day, TID means three times a day, and QID means four times a day. PRN means as needed, QHS means at bedtime, and PO means by mouth. AC means before meals and PC means after meals. A "q" plus a number, like q6h, means every 6 hours. QD and QOD are on The Joint Commission's "Do Not Use" list, according to ISMP.
Here is the whole set in one table.
| Abbreviation | Plain meaning | What it looks like in a day |
|---|---|---|
| BID (b.i.d.) | Twice a day | 2 doses. Twice-a-day guide |
| TID (t.i.d.) | Three times a day | 3 doses. 3-times-a-day guide |
| QID (q.i.d.) | Four times a day | 4 doses. 4-times-a-day guide |
| q4h, q6h, q8h, q12h | Every 4, 6, 8 or 12 hours | 6, 4, 3 or 2 time slots in 24 hours |
| QHS (q.h.s.) | Every night at bedtime | 1 dose, at your bedtime |
| PRN | As needed | Only when needed |
| PO (p.o.) | By mouth | A route, not a time |
| AC / PC | Before meals / after meals | Tied to when you eat |
The National Library of Medicine's MedlinePlus gives the same plain meanings for BID, TID, QID and PRN, and its medical words tutorial traces BID to the Latin "bis in die."
Knowing the word is only half the job, though. In a 2006 study in Annals of Internal Medicine, 70.7% of patients with low literacy could correctly state the instruction "Take two tablets by mouth twice daily." Only 34.7% of them could show how many pills that meant in a day. Even in plain English, the instruction was easier to repeat than to carry out.
Consult your doctor or pharmacist for advice specific to your medications. The clock times below are examples, not instructions.
What Does BID Mean on a Prescription?
BID means twice a day. It is short for the Latin bis in die, and you will see it written as BID, bid or b.i.d. "PO BID" on a label means by mouth, twice a day.
On a clock, a BID schedule could be 8 AM and 8 PM. The letters don't say how many hours apart. BID counts doses, while q12h (every 12 hours) sets the gap. Whether your doses should follow your meals or stay an even 12 hours apart depends on the medicine. Our twice-a-day guide walks through that difference. Your pharmacist can tell you which one your medicine needs.
TID Meaning in Medical Terms
Three times a day is all TID says. It does not say "every 8 hours." That one is written q8h.
Three times a day can fit into your waking hours. For example, a morning, midday and evening dose. Every 8 hours spreads doses across the full 24 hours, which can mean one lands at night. Which one your medicine needs depends on the drug, so ask your pharmacist. The 3-times-a-day guide has a dose-interval calculator. The every-8-hours guide covers round-the-clock timing.
Is QID 4 Times a Day or Every 6 Hours?
QID is four times a day. By definition it is not the same as every 6 hours. QID says how many doses a day. Q6h says how many hours apart, and four doses 6 hours apart put at least one dose in the night. The 4-times-a-day guide shows common ways to set QID times. The every-6-hours guide covers the round-the-clock version.
What Does Q6H Mean? (Q4H, Q8H and Q12H)
Q6H means every 6 hours. The number is the gap between doses, so q4h is every 4 hours and q12h is every 12. Watch for a look-alike: ISMP warns that "q6PM," meant as every evening at 6 PM, can be "Mistaken as every 6 hours". Its advice is to write "6 PM daily" instead.
- q4h is every 4 hours: 6 time slots in 24 hours. See the every-4-hours guide.
- q6h is every 6 hours: 4 time slots in 24 hours. See the every-6-hours guide.
- q8h is every 8 hours: 3 time slots in 24 hours. See the every-8-hours guide.
- q12h is every 12 hours: 2 time slots in 24 hours. See the twice-a-day guide.
These are counts of time slots, not doses you must take. Whether you need to wake up for a night dose is a question for your pharmacist.
PRN Meaning on a Prescription
PRN means as needed. It comes from the Latin pro re nata. A 2018 review in the journal Pharmacy describes PRN medicine as "prescribed in advance, with administration as-needed."
A PRN medicine is taken only when you need it, not on a fixed schedule. PRN can be paired with a time. Read "q6h PRN" piece by piece: every 6 hours, as needed. Ask your pharmacist the shortest gap between doses and the most you can take in one day.
QHS Meaning in a Prescription (and HS)
QHS means every night at bedtime. ISMP's list gives its meaning as "Nightly at bedtime."
The abbreviation ties the dose to your bedtime, not to a clock time. If you work nights and sleep during the day, ask your pharmacist whether your dose should follow your sleep or the clock.
One warning from a safety group: ISMP lists a handwritten "Qhs" as mistaken for "qhr," every hour. Its advice is to write QHS or qhs for bedtime.
For more on shifting schedules, see our night shift medication tips.
What Does PO Mean on a Prescription?
PO is about how you take a medicine, not when. It stands for the Latin per os, "by mouth." So "1 tab PO BID" reads: one tablet, by mouth, twice a day.
The full Latin "per os" is the version to avoid. ISMP says the "os" has been mistaken for left eye, and recommends writing "PO, by mouth, or orally."
AC and PC Meaning in a Prescription
AC means before meals. PC means after meals.
How long before or after a meal? The abbreviation doesn't say, and the right gap depends on the medicine. Our guide to taking medication with or without food covers the common patterns. For "take on an empty stomach," see what that phrase actually means.
Do Not Use Abbreviations List (Joint Commission and ISMP)
Some abbreviations are so easy to misread that safety groups tell prescribers to stop using them. The Institute for Safe Medication Practices (ISMP) keeps a list of abbreviations "reported to ISMP through the ISMP National Medication Errors Reporting Program" that were misread in harmful or potentially harmful errors. It marks the ones on The Joint Commission's shorter "Do Not Use" list, which organizations it covers "must" include on their own lists. QD and QOD are on both.
| Abbreviation | Meant | Has been misread as | Write instead |
|---|---|---|---|
| QD, q.d. | Every day | q.i.d. (four times a day) | daily |
| QOD, q.o.d. | Every other day | qd (daily) or qid (four times a day) | every other day |
| OD, o.d. | Once daily | Right eye (oculus dexter) | daily |
| q1d | Daily | qid (four times a day) | daily |
| q6PM | Every evening at 6 PM | Every 6 hours | 6 PM daily |
| TIW | 3 times a week | 3 times a day, or twice a week | 3 times weekly |
| BIW | 2 times a week | 2 times a day | 2 times weekly |
What does QD mean?
QD means every day, or once daily. ISMP marks it as being on The Joint Commission's list. The problem it reports: the period after the q or the tail of a handwritten q can be read as the letter "i," turning "daily" into q.i.d., four times a day.
What does QOD mean?
QOD means every other day. ISMP says it has been mistaken for daily or for four times a day, "especially if the "o" is poorly written." In Canada, ISMP Canada's "Do Not Use" list flags QD, QOD and OD, and says to use "daily" and "every other day" instead.
If you see QD, QOD or OD on a label or discharge sheet, ask the pharmacist to write the instruction out in words. Our list of questions to ask your pharmacist is a handy script.
How Pillo Helps
The same schedule written two ways can split your day into extra dose times. In a 2011 study in Archives of Internal Medicine, 464 adults aged 55 to 74 planned a day for a made-up 7-drug regimen. When two drugs had the same frequency written differently, "every 12 hours" versus "twice daily," 79.0% did not line them up at the same times.
Once your pharmacist confirms what BID, TID or q8h means for your medicine, you set those doses as alarms at the times you choose. Schedules that change from day to day fit too. The persistent alarm keeps going until you act on the dose. If a parent or child relies on you for their medications, you can add them as dependents and keep their schedules in your app.
Frequently Asked Questions
Is BID the same as every 12 hours?
Not by the letters. BID means twice a day, while q12h means every 12 hours. A twice-a-day label doesn't say how far apart the doses go, so ask your pharmacist whether your medicine needs an even 12-hour gap.
What does q6h PRN mean?
Read it in two parts: q6h is every 6 hours, and PRN is as needed. Put together, it reads "every 6 hours, as needed." Ask your pharmacist for the shortest gap between doses and the daily limit for your medicine.
What does 1 tab PO BID mean?
It means take one tablet by mouth, twice a day. "Tab" is tablet, PO is by mouth, and BID is twice a day. Ask your pharmacist whether to take it with food.
What is the difference between BID, TID and QID?
They count doses per day. BID is twice a day, TID is three times a day, and QID is four times a day.
Why can't doctors write QD anymore?
According to ISMP's list, QD is on The Joint Commission's "Do Not Use" list and has been misread as QID, four times a day. Safety groups recommend writing "daily" instead.
This article is for informational purposes only and does not constitute medical advice. Always consult your doctor or pharmacist before making any changes to your medication routine.
