Prescription Writing and Pharmacokinetics Overview

Question 1

Under what classification is Vicodin scheduled?

Answer 1

Vicodin is a schedule 2 drug, as such it must be prescribed by an in-person visit with the physician, not over the phone.

Question 2

What is special about schedule 5 drugs?

Answer 2

Schedule 5 drugs are frequently OTC, however different states may regulate them differently. Sudafed is available “behind the counter” in Colorado, without a prescription but with ID and in controlled amounts. In OR, it requires a prescription.

Question 3

What are some current requirements for prescribing controlled drugs?

Answer 3

Any prescription for controlled substance requires a DEA number (physician registers with DEA). Schedule 1 drugs may not be prescribed. All schedule 2-4 drugs require a prescription (CO includes schedule 5). Schedule 2 must be in ink in physicians hand writing, cannot be phoned in, cannot be refilled. Schedule 3-5 can be phoned in, may be refilled up to 5 times in 6 months.

Question 4

Two top characteristics of prescription fraud.

Answer 4

1) Handwriting is too legible 2) Directions are written in full, no abbreviations.

Question 5

How many milligrams are in a grain?

Answer 5

64.8 mg per grain

Question 6

How many grams are in an ounce?

Answer 6

28.35 grams per ounce

Question 7

How many milliliters are in a teaspoon? In a tablespoon?

Answer 7

5 ml per tsp, 15 ml per tbsp

Question 8

How many milliliters are in a fluid ounce?

Answer 8

29.56 ml in a fluid ounce

Question 9

What is a simplified definition of pharmacokinetics?

Answer 9

The change in plasma concentration (Cp) over time.

Question 10

What does needing to know the actual plasma concentration of a drug in a patient imply about the drug?

Answer 10

It implies that the drug has a narrow therapeutic window.

Question 11

What is the relationship between the dose of a drug and the resultant Cp?

Answer 11

Cp = (Dose [mg])/(Vd [L]) where Vd (Volume of distribution) is the dilution factor (Assuming that the bioavailability F is 100%)

Question 12

What is the relationship between the steady state Cp and the maintenance dose?

Answer 12

MD/tau = Cpss x CL where MD is the maintenance dose in mg and tau is the time in hours, thus (mg/hr); Cpss is the steady state concentration in mg/L and CL is the clearance rate in L/hr. Thus Cpss [mg/L] = (MD/tau)/CL {mg/hr/L/hr} (Assuming that the bioavailability F is 100%, and that the system is at steady state, rate in = rate out)

Question 13

How is bioavailability calculated?

Answer 13

Bioavailability F = (AUC oral)/(AUC iv) It is the ratio of the amount of drug that gets into the blood stream to the total amount of drug (AUC iv is essentially 100% in the blood stream).

Question 14

Why is rate of absorption of a drug less important than other factors?

Answer 14

If it is an emergency, the drug may be given via IV, and thus its rate of absorption is irrelevant. If it is a long term drug that will be taken for many years, then there is no real rush to get Cp into the therapeutic window and the rate is again irrelevant.

Question 15

What is the Volume of Distribution, Vd?

Answer 15

Vd is an abstract concept best thought of as a dilution factor. Just as each drug will have its own F, it will also have its own Vd as a characteristic of the drug.

Question 16

What is the difference between metabolism and excretion of a drug?

Answer 16

Metabolism eliminates drug activity, excretion eliminates the drug itself. Both equally diminish its effectiveness.

Question 17

What is the implication of drugs that go through cells and in what locations in the body do they need this ability?

Answer 17

There are a few locations in the body where drugs must go through cells and cannot go around them. The gut, the brain, and the placenta all require transport through cells. Thus, if a drug can move through the gut into the body, it can also cross into the brain or ino the placenta and exert effects in those locations. This property may also allow drugs to pass back out of the urine filtrate back into the blood stream.

Question 18

What is a major tactic in drug metabolism that concerns a drug’s ability to cross through cells?

Answer 18

If a drug can be metabolised in such a way that it cannot pass through cells, then it can be eliminated through the urine because it cannot cross back into the blood stream at that point.