Prescriptions are often written in a shorthand that is mostly Latin, mostly abbreviated, and mostly unexplained. BID, TID, QID, PRN and q6h all show up on a prescription as if everyone already knows them. The pharmacist translates them into plain English for the label, but you will still see them on hospital discharge papers, printouts and patient portals, and in the notes you get after a doctor's appointment.
This guide covers what each common prescription abbreviation means, what it means in actual hours, and the handful that are dangerous enough that hospitals are told not to use them at all.
The frequency abbreviations
These say how often. Most come from Latin, which is why the letters do not match the English words.
| Abbreviation | Latin | Plain English | Doses a day |
|---|---|---|---|
| QD (avoid) | quaque die | once a day | 1 |
| BID, BD | bis in die | twice a day | 2 |
| TID, TDS | ter in die, ter die sumendum | three times a day | 3 |
| QID, QDS | quater in die, quater die sumendum | four times a day | 4 |
| QHS, HS (avoid) | quaque hora somni | at bedtime | 1 |
| QOD (avoid) | quaque altera die | every other day | ½ on average |
BID, TID and QID are the usual US forms. BD, TDS and QDS are the usual UK forms. They mean the same thing.
BID, TID and QID are not strict intervals
This is the part people get wrong. "Twice a day" is not automatically "every 12 hours", and "three times a day" is not automatically "every 8 hours". A label that says three times a day usually means three doses spread across the hours you are awake, often around meals. A label that says every 8 hours means a fixed gap around the clock, night included.
For many medications the difference does not matter much. For some, such as certain antibiotics, it does, and that is why a prescriber chooses one wording over the other. The long version is in every 8 hours vs three times a day and twice a day vs every 12 hours. If you want waking-hours times for a times-a-day instruction, the times-a-day schedule spaces them inside your day.
The interval abbreviations: q4h, q6h, q8h
A lowercase q followed by a number and h means "every so many hours". It comes from quaque, "every".
| Abbreviation | Means | Times a day | Hours apart |
|---|---|---|---|
| q4h | every 4 hours | 6 | 4 |
| q6h | every 6 hours | 4 | 6 |
| q8h | every 8 hours | 3 | 8 |
| q12h | every 12 hours | 2 | 12 |
| q24h | every 24 hours | 1 | 24 |
These are strict intervals. q6h starting at 8am is 8am, 2pm, 8pm and 2am, and the 2am dose is part of the instruction, not a mistake. The dose interval calculator gives the clock times for any start, and the hours to times a day calculator converts in either direction, including awkward gaps like every 5 or every 7 hours.
You will also see ranges such as q4–6h, every 4 to 6 hours. The smaller number is the shortest gap allowed. The full explanation of how a range combines with a daily maximum is in every 4 hours as needed.
PRN and the timing words
PRN is pro re nata, "as the occasion arises": take it when you need it, not on a schedule. A PRN instruction should always come with limits, a minimum gap and usually a maximum number of doses in 24 hours. If yours does not, ask the pharmacist what the limits are before you rely on it. The next dose time calculator works out when the label next allows a dose.
The timing abbreviations attach a dose to a meal or to sleep:
| Abbreviation | Latin | Means |
|---|---|---|
| AC | ante cibum | Before meals. Usually on an empty stomach, commonly 30 to 60 minutes before eating; follow the exact wording on your label. |
| PC | post cibum | After meals |
| HS (avoid) | hora somni | At bedtime |
| QHS | quaque hora somni | Every night at bedtime |
| QAM / QPM | Every morning / every evening | |
| STAT | statim | Immediately. Mostly a hospital word. |
And the route abbreviations, which say how the medicine goes in:
| Abbreviation | Means |
|---|---|
| PO (per os) | By mouth |
| SL | Sublingual, under the tongue |
| IM / IV / SC or SQ | Into a muscle / into a vein / under the skin |
| OD / OS / OU | Right eye / left eye / both eyes. Also on error-prone lists, because they get confused with each other and with ear abbreviations. |
The abbreviations hospitals are told not to use
The Joint Commission, which accredits US hospitals, keeps an official "Do Not Use" list of abbreviations that have caused real medication errors. Accredited organizations are expected to keep them off orders and medication documents. The list is short:
| Do not use | Why it goes wrong | Write instead |
|---|---|---|
| U, IU | U read as 0 or 4, IU read as IV or 10 | "units", "international units" |
| QD, Q.D. | confused with QID (four times a day) | "daily" |
| QOD, Q.O.D. | confused with QD or QID | "every other day" |
| Trailing zero (5.0 mg) | decimal point missed, read as 50 mg | 5 mg |
| No leading zero (.5 mg) | decimal point missed, read as 5 mg | 0.5 mg |
| MS, MSO4, MgSO4 | morphine and magnesium sulfate confused | the full drug name |
Look at the two zero rules for a moment, because they are about factors of ten. A missed decimal point in ".5 mg" is a dose ten times too large. That is why a well-written prescription always says 0.5 and never 5.0.
The Institute for Safe Medication Practices (ISMP) keeps a much longer list of error-prone abbreviations that it recommends avoiding, including HS (which can be read as "half-strength" as well as "bedtime") and the eye and ear abbreviations above.
What this means for you
You are not going to rewrite your prescriptions, but two habits help:
- If you see QD or QOD on anything, check it. On a discharge summary or a handwritten note, "QD" for once daily and "QID" for four times a day are one smudge apart. Ask the pharmacist to confirm.
- Read decimals carefully. If a dose looks like it has an unusual number of zeros, or no zero before the decimal point, ask. It takes thirty seconds and it is the kind of question pharmacists are glad to get.
Reading a full instruction (a "sig")
Pharmacists call the instruction line the sig, from signa, "label it". Put the pieces together and a sig reads like a sentence:
| Sig | In plain English |
|---|---|
| 1 tab PO BID | One tablet by mouth twice a day |
| 2 gtt OU TID | Two drops in both eyes three times a day (gtt is guttae, drops) |
| 1–2 tabs PO q4–6h PRN pain, max 8/24h | One or two tablets by mouth every 4 to 6 hours as needed for pain, no more than 8 tablets in 24 hours |
| 1 cap PO QHS | One capsule by mouth at bedtime |
When the label you take home is in plain English, it was translated from a line like these. If the plain English and what the doctor told you do not match, the pharmacist can see the original and tell you which is right.
Turning the label into reminders
Once you know what the shorthand means, the remaining problem is doing it, every day, at the right times. Writing "TID" into a phone alarm does not help much. Three alarms tied to breakfast, lunch and dinner do.
Doz is a free iPhone medication reminder built for this: you enter each medication with its times, or anchor it to breakfast, lunch or dinner, and it reminds you when a dose is due. It handles schedules that are not daily, such as every other day or on-and-off cycles, and flags a medication as low in the app before the bottle runs out. With Doz Pro, reminders can ring even when the phone is on silent through Critical Alerts, which helps when a q8h dose falls at night.
Frequently Asked Questions
What does BID mean on a prescription?
BID means twice a day, from the Latin bis in die. It usually means a morning and an evening dose, often with meals, rather than strictly every 12 hours. If the timing matters for your medication, the prescription will normally say "every 12 hours" or "q12h" instead. In the UK the same instruction is written BD.
What is the difference between TID and q8h?
TID means three times a day, usually spread across your waking hours. q8h means every 8 hours around the clock, which puts one dose overnight if you start in the morning. Both are three doses a day, but the spacing is different. If a label says q8h, keep to the interval and ask your pharmacist before moving a dose.
What does PRN mean?
PRN means "as needed", from the Latin pro re nata. You take the medicine when you have the symptom, not on a fixed schedule. A PRN instruction should come with a minimum gap between doses and usually a maximum number of doses in 24 hours. If yours does not, ask the pharmacist what the limits are.
Why is QD on the Do Not Use list?
QD, for once a day, has been misread as QID, which is four times a day, especially when handwritten. That turns one dose into four. The Joint Commission put it on its official Do Not Use list, and accredited US hospitals are expected to write "daily" instead. QOD, for every other day, is on the list for the same reason.
What does q6h mean?
q6h means every 6 hours, which is four times a day. The q is from the Latin quaque, "every", and h is hours. It is a strict interval: starting at 8am, the doses are at 8am, 2pm, 8pm and 2am.
The short version
Prescription abbreviations are a small vocabulary. BID, TID and QID are two, three and four times a day, usually in waking hours. q6h and q8h are strict intervals. PRN is as needed, with limits. QD, QOD, trailing zeros and missing leading zeros are all on the list of things that cause errors, so if you see them, check. And whatever the abbreviation, the pharmacist who filled it can read the original prescription and explain it.
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