I'm a bit late on this discussion, but I write prescriptions every day. Various posters are correct. A prescription has several parts:
1. What it is (e.g., prednisone tabs, 10 mg). One should use the generic term, but often the brand name is easier
2. What to do with it and for how long: e.g. one tablet by mouth twice each day. As several posters have noted, this is written in physician shorthand. For example, shorthand for the above would be "I po BID." If it only comes as an oral pill, the "po" is left out. This part of the prescription is traditionally begun with the abbreviation "Sig," standing for "let it be so inscribed" in Latin. Believe it or not, at one time it was common not to put the directions on the bottle, so the "Sig" tells the pharmacist to do so (in plain English).
3. How much to dispense. The prescriber often puts a line after the number so that it cannot be altered. As several posters have noted, the pharmacist is required by law to dispense this amount--no more, no less.
4. Whether to allow refills--if so, how many and for how long. Maximum is one year.
5. Whether to allow substitution of generic drugs: this is usually a check-box on the form. If not checked, substitution is allowed.
So a typical prescripton for an antibiotic like cephlexin (Keflex) would look like this:
Rx: Cephlexin caps 250 mgs
Sig: I BID x 10d
Disp: #20---
Refills: 0
The pharmacist always checks the math to see if the number of pills matches how long the person is supposed to take it. The prescription cannot be changed in any way without checking with the physician first.