In selecting a vehicle for a liquid prescription the prescriber must consider its solvent action, compatibility, taste, odor, color, and cost.

Solvent Action

If it is desired to administer a drug in solution the vehicle must naturally be one that will dissolve that particular substance. This matter is discussed at some length elsewhere, but the following general rules will bear repetition:

Salts (metallic or alkaloidal) and gums are usually more soluble in aqueous liquids. Alkaloids, oils, resins, oleoresins, and some coal-tar products are more soluble in alcoholic liquids.

If it is desired to administer a drug in suspension the vehicle must naturally be one that will not dissolve that particular drug.

Compatibility

This is taken up in detail on page 397.

Taste

The matter of taste is one that cannot be overlooked. Unpleasant developments, as nausea and vomiting, may result from a disagreeable preparation. The patient may, if an adult, refuse to take the medicine, and, if a child, the struggle to administer the dose may often more than counterbalance the good effect of the remedy. To a certain extent the people demand to be cured not only quickly but pleasantly, and if one physician can not or will not do it another will.

Odor

Disagreeable odors are usually unnecessary and possess all the bad features mentioned in regard to taste.

Color

Under this head might be included the matter of general appearance. When it is possible to do so without a sacrifice of quality; it is certainly better to order a preparation that will be elegant in appearance. Color often has some psychic effect. It is very common to hear a patient rail against the doctor who"did not give me anything but water."

Cost

This is often a factor. Unless there is a definite reason for so doing, an expensive vehicle should not be employed. The remark is common when a doctor writes for an expensive proprietary q. s.:"I do not think that it is much good, but it makes a good vehicle." The patient has to pay for that good vehicle.

The following very imperfect list of vehicles are among those worthy of special mention:

Water should be considered as the one great vehicle, and employed unless there is some reason against its use. Make it the agent of first thought in prescribing solutions, and elaborate from it.

Distilled Water is valuable when it is desired to use a vehicle free from matter that might result in chemical action, as in prescribing silver nitrate, etc.

Peppermint-water, Cinnamon-water, and Chloroform-water are valuable where there is nausea and for disguising the taste of many salts.

Aromatic Elixir, which contains about 25 per cent. Alcohol, is a very valuable vehicle. It is a good solvent, has a pleasant taste and odor, and the alcoholic content makes it a good preservative. It is particularly desirable in prescribing such salts as the bromides, alkaloidal salts, etc.

Syrup of Tolu, Syrup of Lemon, Syrup of Wild Cherry, and Syrup of Squills are used particularly in prescribing cough mixtures. They are used as the vehicle alone or with water.

Mucilage of Acacia, Syrup of Acacia, and Syrup are frequently employed when insoluble agents are to be suspended in a liquid.

Compound Tincture of Gentian and Compound Ticture of Cinchona are used as vehicles in prescribing bitter tonics. They, of course, are not compatible with ferric salts. They contain about 50 per cent. alcohol, which must be taken into consideration.

Tincture of Sweet Orange and Compound Spirit of Orange are good alcoholic vehicles, particularly for such drugs as creosote, guaiacol, etc.

Alcohol is an indispensable part of many other vehicles and is frequently used alone, as when prescribing agents requiring its solvent effect or preservative action.

Glycerin is frequently employed alone or in connection with other vehicles, particularly for such agents as phenol, iodine, tannic acid, etc.

Soap Liniment is a useful vehicle in prescribing liniments. To it can be added such agents as chloroform, menthol, etc.

Liquid Petrolatum is the vehicle of common choice for prescribing oil-soluble drugs as sprays, particularly such agents as camphor, menthol, and the volatile oils.

Sugar of Milk is the most commonly used vehicle in prescribing powders. It is an inert, white powder, without odor and of a mildly sweet taste.

Aromatic Powder is a finely powdered mixture of spices, and is often useful in disguising taste and odor and in giving bulk and color to powders.

Petrolatum for ointments occupies the place that water does for solutions. It should be the vehicle used unless there is some reason to the contrary. It is inert, odorless, clean, and permanent.

Ointment of Rose-water is an excellent ointment base.

Hydrous Wool-fat is used as a vehicle in ointments when absorption of the active agent is desired.