Anatomy of a Prescription

I still remember the first time I wrote a medication order. Tylenol 650mg PO/PR q6h PRN fever. I ran all over the hospital, looking for my supervising resident to ask if I had done it correctly. Indeed I had...my first success.

Everybody has had the experience of going to the doctor's and coming home with a prescription written illegibly with 10 sanscrit-like letters, which after filling at the pharmacy, somehow becomes 10 words on the actual label. Ever wondered what those abbreviations mean?

The first word of a prescription is always the name of the drug. Tylenol. Acetaminophen is the generic name of tylenol (Tylenol is the brand name). Brand name drugs are manufactured by the company that did the research and "invented" the drug. Since they invented the drug, they are free to patent it and make money off of their research. But after 15 or so years, the patent expires and other companies are free to make the drug. They usually do so at a much cheaper price since this opens the door to the free market and competition.

Usually when a script is written, the generic form will be given, even if the prescription is written for the brand name unless "no substitutions allowed" is specifically indicated.
For most purposes, the brand name and generic versions are biologically identical. (There may actually be chemical differences, but this usually does not effect how the drug works).

After the name of the drug comes the dosage. Most drugs are dosed in milligrams. Obviously the dosage prescibed to you depends on many things: how potent the drug is, how much medicine you actually need, etc.

The next line includes how the medication should be given, or the route of administration. This is where the ancient abbreviations come in.

P.O. - by mouth
I.V. - by intravenous
I.M. - intramuscularly (a shot in the butt or arm)
S.C. - subcutaneously (a shot just under the skin)
S.L. - sublingual (under the tongue)
P.R. - per rectum (rectal suppository)

Obviously most outpatient prescriptions are PO since this is the easiest, but there are advantages to the other routes of administration. IV medications are usually more potent and start working much faster than pills. IM medications are not quite as fast to work as IV meds, but there are times when you might not want to start an IV on the patient. Rectal suppositories are good for when patients aren't allowed to take things by mouth (NPO, or Nothing PO).

After the route of administration comes the frequency, or how often the medication should be taken. Here are some more abbreviations:

QD - daily
BID - twice daily
TID - three times daily
QID - four times daily
QOD - every other day

Alternatively, the letter "q" can be used to mean "every". So "q6hours" means "every 6 hours). This convention has affectionately been applied to describe call schedules. So if you ever hear an intern say, "the call schedule is q3", it means that they are on call every third night.

After the frequency, physicians often will write "PRN". This means "as needed for ___". For example PRN pain means as needed for pain. So if there is no pain, obviously no medication is needed.

The last line of the prescription is usually just the amount of medication to dispense. It is frequently written as a number sign (#) followed by a number, signifying the number of pills to dispense. And then of course the bottom of the prescription usually has an area where the number of refills can be written.

So Tylenol 650mg PO/PR q6h PRN fever is just fancy way of prescribing tylenol 650mg by mouth or rectal suppository every six hours as needed for fever.

There you go, now you know what your doctor is prescribing for you. Now if only you could read the handwriting...