Drug, prescription: A drug requiring a prescription, as opposed to an over-the-counter drug, which can be purchased without one. The word "prescription" comes from the Latin "praescriptus" compounded from "prae", before + scribere, to write = to write before. Historically, a prescription was written before the drug was prepared and administered.

A prescription has several parts. There are:

• The superscription (or heading) with the symbol R or Rx which stands for the word Recipe, meaning (in Latin) to take;
• The inscription which contains the names and quantities of the ingredients;
• The subscription or directions for compounding the drug; and
• The signature which is often preceded by the sign s. standing for signa, mark, giving the directions to be marked on the container.

Seen on a prescription, b.i.d. means twice (two times) a day. It is an abbreviation for "bis in die" which in Latin means, not too surprisingly, twice a day. The abbreviation b.i.d. is sometimes written without a period either in lower-case letters as "bid" or in capital letters as "BID". 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:

• q.d. (qd or QD) is once a day; q.d. stands for "quaque die" (which means, in Latin, once 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."