Dosage
The amount of medication to be administered is referred to as the dose. The study of dosage and the criteria that influence it is called posology. The doses given in the United States Pharmacopeia and National Formulary (USP-NF) are average therapeutic doses and are known as "usual adult doses." The following terms are used in connection with doses.

THERAPEUTIC DOSE.-Therapeutic dose is also referred to as the normal adult dose, the usual dose or average dose. It is the amount needed to produce the desired therapeutic effect. This therapeutic dose is calculated on an average adult of 24 years who weighs approximately 150 pounds.

DOSAGE RANGE.-Dosage range is a term that applies to the range between the minimum and maximum amounts of a given drug required to produce the desired effect. Many drugs (such as penicillin) require large initial doses that are later reduced to smaller amounts. Closely associated with "dosage range" are the terms minimum dose (the least amount of drug required to produce a therapeutic effect), maximum dose (the largest amount of drug that can be given without reaching the toxic effect), and toxic dose (the least amount of drug that will produce symptoms of poisoning).

MINIMUM LETHAL DOSE.-Minimum lethal dose is the least amount of drug that can produce death.

Factors Affecting Dosage

The two primary factors that determine or influence the dosage of a medication are the age and weight of the patient.

AGE.-Age is the most common factor that influences the amount of drug to be given. An infant requires a lower dose than an adult. Elderly patients may require a higher or lower dose than the average dose, depending upon the action of the drug and the condition of the patient.

The rule governing calculation of pediatric (child's) doses, Young's Rule, is expressed as follows:

The age in years of the child is the numerator, and the age plus 12 is the denominator. This fraction is multiplied by the normal adult dose.

WEIGHT.-In the calculation of dosages, weight has a more direct bearing on the dose than any other factor, especially in the calculation of pediatric doses. The rule governing calculation of pediatric doses based on weight is Clark's Rule, expressed as follows:

The child's weight in pounds is the numerator, and the average adult weight (150 pounds) is the denominator. This fraction is multiplied by the adult dose.

OTHER FACTORS THAT INFLUENCE DOSAGE.-Other factors that influence dosage include the following:

Sex-Females usually require smaller doses than males.

Race-Black individuals usually require larger doses, and Asians require smaller doses than Caucasians.
Occupation-Persons working in strenuous jobs may require larger doses than those who sit at a desk all day.
Habitual use-Some patients must take medications continuously, causing their bodies to build up tolerance to the drug. This tolerance may require larger doses than their initial doses to obtain the same therapeutic effect.

Time of administration-Therapeutic effect may be altered depending upon time of administration (e.g., before or after meals).

Example: The adult dose of aspirin is 650 mg. What is the dose for a 3-year-old child?

Example: The adult dose of aspirin is 650 mg.
What is the dose for a child weighing 60 pounds? 240

Frequency of administration-Drugs given frequently may need a smaller dose than if administered at longer intervals.
Mode of administration-Injections may require smaller doses than oral medications.

Methods of Administering Drugs
Drugs may be introduced into the body in several ways, each method serving a specific purpose.

ORAL.-Oral administration of medications is the most common method. Among the advantages of administering medication orally (as opposed to other methods) are the following:

Oral medications are convenient.
Oral medications are cheaper.
Oral medications do not have to be pure or sterile.

Awide variety of oral dosage forms is available. Oral medication administration may be disadvantageous for the following reasons:

Some patients may have difficulty swallowing tablets or capsules.

Oral medications are often absorbed too slowly.
Oral medications may be partially or completely destroyed by the digestive system.

Other methods of administration closely associated with oral administration are sublingual and buccal. Sublingual drugs are administered by placing the medication under the tongue. The medication is then rapidly absorbed directly into the blood stream. An example of a sublingual drug is nitroglycerin sublingual tablets (for relief of angina pectoris).

Buccal drugs are administered by placing the medication between the cheek and gum. Buccal drugs, like sublingual drugs, are quickly absorbed directly into the blood stream. An example of a drug that may be given buccally is the anesthetic benzocaine.

PARENTERAL.-Parenteral medications are introduced by injection. All drugs used by this route must be pure, sterile, pyrogen-free (pyrogens are products of the growth of microorganisms), and in a liquid state. There are several methods of parenteral administration, including subcutaneous, intradermal, intramuscular, intravenous, and intrathecal or intraspinal.

Subcutaneous.-The drug is injected just below the skin's cutaneous layers. Example: Insulin.

Intradermal.-The drug is injected within the dermis layer of the skin. Example: Purified protein derivative (PPD).

Intramuscular.-The drug is injected into the muscle. Example: Procaine penicillin G.

Intravenous.-The drug is introduced directly into the vein. Example: Intravenous fluids.

Intrathecal or Intraspinal.-The drug is introduced into the subarachnoid space of the spinal column. Example: Procaine hydrochloride.

INHALATION.-Inhalation is a means of introducing medications through the respiratory system in the form of a gas, vapor, or powder. Inhalation is divided into three major types: vaporization, gas inhalation, and nebulization.

Vaporization.-Vaporization is the process by which a drug is changed from a liquid or solid to a gas or vapor by the use of heat (such as in steam inhalation).

Gas Inhalation.-Gas inhalation is almost entirely restricted to anesthesia.

Nebulization.-Nebulization is the process by which a drug is converted into a fine spray by the use of compressed gas.

TOPICAL.-Topical drugs are applied to a surface area of the body. Topically applied drugs serve two purposes:

Local effect: The drug is intended to relieve itching, burning, or other skin conditions without being absorbed into the bloodstream.
Systemic effect: The drug is absorbed through the skin into the bloodstream.

Examples of topical preparations are ointments, creams, lotions, and shampoos.

RECTAL.-Drugs are administered rectally by inserting them into the rectum. The rectal method is preferred to the oral route when there is danger of vomiting or when the patient is unconscious, uncooperative, or mentally incapable. Examples of rectal preparations are suppositories and enemas.

VAGINAL.-Drugs are inserted into the vagina to produce a local effect. Examples of vaginal preparations are suppositories, creams, and douches.