Guide to Good Prescribing - A Practical Manual
(1994; 115 pages) [French] [Spanish] View the PDF document
Table of Contents
View the documentAcknowledgments
View the documentWhy you need this book
Open this folder and view contentsPart 1: Overview
Close this folderPart 2: Selecting your P(ersonal) drugs
View the documentChapter 2. Introduction to P-drugs
View the documentChapter 3. Example of selecting a P-drug: angina pectoris
Close this folderChapter 4. Guidelines for selecting P-drugs
View the documentStep i: Define the diagnosis
View the documentStep ii: Specify the therapeutic objective
View the documentStep iii: Make an inventory of effective groups of drugs
View the documentStep iv: Choose an effective group according to criteria
View the documentStep v: Choose a P-drug
View the documentChapter 5. P-drug and P-treatment
Open this folder and view contentsPart 3: Treating your patients
Open this folder and view contentsPart 4: Keeping up-to-date
Open this folder and view contentsAnnexes
View the documentBack Cover
 

Step v: Choose a P-drug

There are several steps to the process of choosing a P-drug. Sometimes short-cuts are possible. Don't hesitate to look for them, but do not forget to collect and consider all essential information, including existing treatment guidelines.

Choose an active substance and a dosage form

Choosing an active substance is like choosing a drug group, and the information can be listed in a similar way. In practice it is almost impossible to choose an active substance without considering the dosage form as well; so consider them together. First, the active substance and its dosage form have to be effective. This is mostly a matter of kinetics.

Although active substances within one drug group share the same working mechanism, differences may exist in safety and suitability because of differences in kinetics. Large differences may exist in convenience to the patient and these will have a strong influence on adherence to treatment. Different dosage forms will usually lead to different dosage schedules, and this should be taken into account when choosing your P-drug. Last, but not least, cost of treatment should always be considered. Price lists may be available from the hospital pharmacy or from a national formulary (see Table 4, Chapter 3 for an example).

Keep in mind that drugs sold under generic (nonproprietary) name are usually cheaper than patented brand-name products. If two drugs from the same group appear equal you could consider which drug has been longest on the market (indicating wide experience and probably safety), or which drug is manufactured in your country. When two drugs from two different groups appear equal you can choose both. This will give you an alternative if one is not suitable for a particular patient. As a final check you should always compare your selection with existing treatment guidelines, the national list of essential drugs, and with the WHO Model List of Essential Drugs, which is reviewed every two years.

Choose a standard dosage schedule

A recommended dosage schedule is based on clinical investigations in a group of patients. However, this statistical average is not necessarily the optimal schedule for your individual patient. If age, metabolism, absorption and excretion in your patient are all average, and if no other diseases or other drugs are involved, the average dosage is probably adequate. The more your patient varies from this average, the more likely the need for an individualized dosage schedule.

Recommended dosage schedules for all P-drugs can be found in formularies, desk references or pharmacology textbooks. In most of these references you will find rather vague statements such as ‘2-4 times 30-90 mg per day’. What will you choose in practice?

The best solution is to copy the different dosage schedules into your own formulary. This will indicate the minimum and maximum limits of the dosage. When dealing with an individual patient you can make your definitive choice. Some drugs need an initial loading dose to quickly reach steady state plasma concentration. Others require a slowly rising dosage schedule, usually to let the patient adapt to the side effects. Practical aspects of dosage schedules are further discussed in Chapter 8.

Box 4: General characteristics of dosage forms

Systemic dosage forms


oral (mixture, syrup, tablet (coated, slow-release), powder, capsule)


sublingual (tablet, aerosol)


rectal (suppository, rectiol)


inhalation (gasses, vapour)


injections (subcutaneous, intramuscular, intravenous, infusion)

Local dosage forms


skin (ointment, cream, lotion, paste)


sense organ (eye drops/ointment, ear drops, nose drops)


oral/local (tablets, mixture)


rectal/local (suppository, enema)


vaginal (tablet, ovule, cream)


inhalation/local (aerosol, powder)

Oral forms


efficacy:

(-) uncertain absorption and first-pass metabolism, (+) gradual effect


safety:

(-) low peak values, uncertain absorption, gastric irritation


convenience:

(-)? handling (children, elderly)

Sublingual tablets and aerosols


efficacy:

(+) act rapidly, no first-pass metabolism


safety:

(-) easy overdose


convenience:

(-) aerosol difficult to handle, (+) tablets easy to use

Rectal preparations


efficacy:

(-) uncertain absorption, (+) no first-pass metabolism, rectiol fast effect


safety:

(-) local irritation


convenience:

(+) in case of nausea, vomiting and problems with swallowing

Inhalation gasses and vapours


efficacy:

(+) fast effect


safety:

(-) local irritation


convenience:

(-) need handling by trained staff

Injections


efficacy:

(+) fast effect, no first-pass metabolism, accurate dosage possible


safety:

(-) overdose possible, sterility often a problem


convenience:

(-) painful, need trained staff, more costly than oral forms

Topical preparations


efficacy:

(+) high concentrations possible, limited systemic penetration


safety:

(-) sensitization in case of antibiotics, (+) few side effects


convenience:

(-) some vaginal forms difficult to handle

Choose a standard duration of treatment

When you prescribe your P-drug to a patient you need to decide the duration of the treatment. By knowing the pathophysiology and the prognosis of the disease you will usually have a good idea of how long the treatment should be continued. Some diseases require life-long treatment (e.g. diabetes mellitus, congestive cardiac failure, Parkinson's disease).

The total amount of a drug to be prescribed depends on the dosage schedule and the duration of the treatment. It can easily be calculated. For example, in a patient with bronchitis you may prescribe penicillin for seven days. You will only need to see the patient again if there is no improvement and so you can prescribe the total amount at once.

If the duration of treatment is not known, the monitoring interval becomes important. For example, you may request a patient with newly diagnosed hypertension to come back in two weeks so that you can monitor blood pressure and any side effects of the treatment. In this case you would only prescribe for the two week period. As you get to know the patient better you could extend the monitoring interval, say, to one month. Three months should be about the maximum monitoring interval for drug treatment of a chronic disease.

Summary

How to select a P-drug

i. Define the diagnosis (pathophysiology)

ii. Specify the therapeutic objective

iii. Make an inventory of effective groups

iv. Choose a group according to criteria



efficacy

safety

suitability

cost


Group 1






Group 2






Group 3





v. Choose a P-drug



efficacy

safety

suitability

cost


Drug 1






Drug 2






Drug 3





Conclusion:

Active substance, dosage form:


Standard dosage schedule:


Standard duration:

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