This section is from the book "Practical Materia Medica And Prescription Writing", by Oscar W. Bethea. Also available from Amazon: Practical Materia Medica and Prescription Writing.
Several general methods of arrangements might be suggested, but no one fixed rule will apply to all cases.
The items may be arranged in the order of their importance, somewhat as basis, adjuvant, corrective, vehicle; but in many instances the basis may consist of more than one drug of practically equal importance, or it is sometimes impossible to definitely fix the items under these heads, as they may merge into each other or one agent may equally belong to more than one class.
The items may be arranged with the solids first and the. liquids last, these respectively arranged as far as possible in the order of their importance.
Where a prescriber is competent to do so, it is often better to list the ingredients in the order in which they should be handled. For example:
Tinct. Ferri Chloridi, | |
Acidi Phosph. Dil., | |
Tinct. Nucis Vomicae........................................ | āā |
Syr. Pruni Virg.................................................. | q. s. |
M.
In this instance the Diluted Phosphoric Acid should be added to the Tincture of Ferric Chloride before the next item, which contains tannic acid, is added, else iron tannate will be formed and an unsightly mixture result.
Another example would be:
Acetanilidi........ | gr. lxxx | |
Spir. Ammon. Arom...... | ||
Caffeinae Citratae......... | gr. xxx | |
Sodii Bromidi.......... | gr. clx | |
Elix. Aromatici............ | q. s. |
M.
Here the Acetanilid should be dissolved in the Aromatic Spirit of Ammonia and the Citrated Caffeine and Sodium Bromide should be dissolved in the Aromatic Elixir, and the two solutions then carefully mixed.
Sometimes special instructions may be given as in the following:
Phenolis............................................................ | gr. v | |
Chlorali Hydrati.................................................... | āā | gr. xx |
M. et adde | ||
Ung. Aq. Rosae........................................................ | q. s. |
Here it is desirable to mix the first ingredients which liquefy and then incorporate them with the vehicle.
Good judgment should always govern the arrangement in the entire matter of prescription writing, but the prescriber can usually profit by bearing in mind the following suggestions:
It is usually best to have the most important ingredients first.
It is usually best to have the solids first.
If for any reason a special order of mixing is desired, it may often be indicated by arrangement.
The vehicle or diluent should always be last and practically always q. s. to make up to the total amount desired.
 
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