Difference Between TID, q8h, and PRN Dosing

Let’s start with a Wikipedia reference of medical abbreviations. A lot of these terms deal with the frequency of taking medications. Now let’s analyze some common pairs: BID (twice a day) vs q12h (every 12 hours); TID (thrice a day) vs q8h (every 8 hours). There is certainly a difference!

I like to think of BID and TID regimens as those which occur when the patient is awake. TID, in particular, tends to follow a breakfast-lunch-dinner schedule (0800, 1400, and 2000 hours, for example). Hourly based regimens (q4h, q6h, q8h, etc) are given strictly in those intervals around-the-clock. This is more useful for medications which require therapeutic level considerations or those which should not be given too frequently. As an example, 1 gram IV vancomycin q12h.

Medications which are given PRN (“as needed”) should be written with reasons. For example, for inpatients with intermittent nausea, I’ve written for 4 mg IV Zofran TID (three times per day) PRN after meals for nausea. By giving a scenario (after meals), I’m avoiding three consecutive doses in rapid succession. In the case of analgesics (Norco 5-325 mg PO q6h PRN for pain) or anti-anxiety meds (Ativan 1 mg IV q6h PRN for anxiety). By using “q6h”, I ensure that the patient receives no more than one dose every 6 hours.

Additional measures we consider when writing medications are holding parameters to avoid giving a medication which could worsen the patient’s clinical status. For example, in the Ativan example, we could “hold dose if excessively sedated.” Above all else, the nurses and pharmacists I’ve worked with tend to be excellent about communicating concerns with the residents/attendings when they’re concerned about medication doses and/or frequencies.

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  1. If script order is written as TID, with NO times for dosing can you legally give the med say at 4 hour intervals or 6 hour intervals or does TiD mean it has to be given at 8 hour intervals

    • For me, TID implies three times during “awake” hours, but the interval is not clear. In situations where the dosing interval is important (e.g., antibiotics), “every 8 hours (q8h)” is more appropriate.

  2. Hi. Just wanted to ask? What if the doctor wrote down Endone PRN QID. Is there a specific number of hour gap between the doses? I know QID is 4x a day, but is there like a gap?

    • Personally, for PRN analgesic medications (especially narcotics) like Endone, I’ll always put some sort of time interval. QID is nonspecific. I’d ask for clarification.


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