In prescriptions all parts of names are in the genitive.

In the prescription

the druggist is told to

Take thou of (the) Tincture of Opium one fluidounce.

So that the words Tinctura and Opium must both be put in the Latin genitive case to express the proper meaning.

The prescriber should remember the suggestion that a good abbreviation is better than a bad termination; so, when in doubt, abbreviate. Spir. is better than Spiritae.