A dictionary says that "ana tres uncias" was used by the Roman doctor Celsus. I googled this and didn't find anything. Was this Greek preposition used in Latin?
I looked up the phrase and found it in the Latin-French Dictionnaire Gaffiot. Here's the entry:
(1) ănă (ἀνά), prép. acc. [à partir de Végèce] : ana tres uncias Veg. Mul. 1, 20, 2, par trois onces.
The entry notes that it is a usage "since Vegetius" (who wrote in the 4th c. AD). The reference is to one of his more obscure works on veterinary medicine: Digesta artis mulomedicinae. Vegetius does not appear to be in the Packhum corpus.
Here's the full passage from I.20 in the entry, with the relevant parts highlighted:
Alia compositio vehementior.
 Est et alia compositio suffimentorum ad morbos prohibendos, sumptuosior quidem sed ut putatur utilior. Sulfuris vivi libr. I, bituminis Iudaici libr. I, opopanacis unc. VI, acanthi unc. VI, galbani libr. I, castorei selib., aeris crudi unc. VI, salis Hammoniaci unc. II, salis Cappadoci unc. III, cornu cervini, lapidis gagatis masculi, lapidis gagatis feminae ana unc. III,  lapidis haematitis unc. II, lapidis sideritis, lapidis argyritis ana unc. I, equuleos id est caballiones marinos numero VII, caudas marinas, ungues marinos ana numero VII, uvae marinae unc III, medullae cervinae, cedriae, picis liquidae ana pondera III, ossa sepiae numero VII, auri semunciam, ballucae siliquam:  et haec universa commixta atque succensa odore suo morbis tam hominum quam animalium resistunt et daemones fugant, grandinem prohibere et aerem defaecare dicuntur. Sed si memoratos lapides aut invenire nequiveris aut enormitate pretii ab emptione cessaveris, reliqua efficaciter prosunt.
The one part that remains confusing to me is what this preposition (obviously borrowed from Greek) means in context. The French translation ("par trois ounces") doesn't seem terribly helpful. I'm also confused by the second occurrence ("ana numero VII"), which appears to take the ablative instead of the accusative.
The Greek adverb ἀνά can be used "distributively with numerals" to mean "at X amount each." What's strange, though, is that there seems to be a contrast between a simple ounce value ("lapidis haematitis unc. II") and an "ana" ounce value ("lapidis argyritis ana unc. I"). One possible meaning is that it can go "up to" that amount.
Might you have the reference wrong? Either that or the dictionary is wrong. There is no ana tres uncias in Celsus.
There is, however, ac tres uncias in Suetonius. Perhaps there was a mistake with a manuscript that editions have since emended.
That said, the only time you get the Greek preposition ana used in Latin is when it's they're using Greek, such as when Varro mentions ἀνὰ λόγον. It is not otherwise a widespread or common preposition. Readers would know that it's Greek, not Latin.
There's also ana in Philumenus, but that's Greek, too.