@Christian_Reich: Thanks for discussing this.
Ad 1:
We do not need a common concentration unit, we just need a common denominator unit. Let’s take a look to a very similar product 42799262 “Benzyl Alcohol 0.1 ML/ML / Pramoxine hydrochloride 10 MG/ML Topical Spray [Itch-X]”. It’s a spray instead of a gel but has the same concentration. Here we have the common denominator mL and we can easily deduce that we have per mL 0.1mL Alcohol and 10mg Pramoxine hydrochloride.
We could do the same for the topical gel. Alternatively, we could provide the same in mg instead of mL: It’s 0.1mg/mg Alcohol or (applying roughly 1g=1mL) 0.0001mL/mg Alcohol and 0.01mg/mg Pramoxine hydrochloride. The percentages in RxNorm make these conversions very easy.
We should chose that denominator unit which fits to the “sales unit”. Eventually, we need the total amount of the active ingredients for dosage calculations. A google search (for example http://www.americanotc.com/mobile.php?seller=AmericanOTC&navt0=a&navc0=&Tsearch=&searchc=&navt1=92837&navt2=96612) shows that both products are sold with the weight unit “oz”: The topical gel is prescribed in 1.25oz, which is appr. 36mg. Thus, we will prefer mg for the denominator units in the DRUG_STRENGTH table.
If we provide 36 in the quantity field of the DRUG_EXPOSURE table, we can calculate the total amount of active ingredients:
36 x 0.0001mL = 0.0036mL Benzyl Alcohol and 36 x 0.01mg = 0.36mg Pramoxine hydrochloride.
Ad 2:
The convention that the unit refers to tablets for solid forms and mL for liquids is fine. But this convention must be consistently applied in the DRUG_STRENGTH table. For solid forms we use the amount_value, the denominator unit is not applicable (conceptual “tablets” or “pieces”).
The problem of divergent units typically applies for gels and sprays. In PharMetrics Plus examples are some prescriptions for “Adapalene Topical Cream“ (quantity in g, strength in ml), “Dapsone Topical Gel” (quantity in g, strength in mg) and “Ipratropium Nasal Spray” (quantity in mL, strength in {actuat}).
Many of them should be easy to fix, for example g to mg or g to mL. Some may need manual research.
There is one additional use case to consider: Patches have a dose release over time. However, we can map this to the current OMOP attributes with the following convention: Put the number of patches in quantity. In the DRUG_STRENGTH table put the release dose in the nominator and 1 hour in the denominator.
Apart from that there are some more particularities (for example release dose per cm²), but they are rarely used and we can neglect them (at least for the time being).
Do you need more information?