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q.d. (on prescription): Seen on a prescription, q.d. (or qd) means one a day (from the Latin quaque die). The abbreviation is sometimes written without a period in capital letters as "QD". However it is written, it is one of a number of hallowed abbreviations of Latin terms that have been traditionally used in prescriptions to specify the frequency with which medicines should be taken.

Other examples include:

b.i.d. (bid or BID) is twice a day; b.i.d.. stands for "bis in die" (which means, in Latin, twice a day).
t.i.d. (or tid or TID) is three times a day ; t.i.d. stands for "ter in die" (in Latin, 3 times a day).
q.i.d. (or qid or QID) is four times a day; q.i.d. stands for "quater in die" (in Latin, 4 times a day).
q_h: If a medicine is to be taken every so-many hours, it is written "q_h"; the "q" standing for "quaque" and the "h" indicating the number of hours. So, for example, "2 caps q4h" means "Take 2 capsules every 4 hours


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Note added at 12 mins (2006-10-21 03:24:17 GMT)
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I mean q.d (seen on prescription) means one a day

b.i.d. o bid twice daily y proviene del Latín "bis die"
q.d. o qd each day y proviene del ´Latín "quaque die"