A prescription allows the following preparation: R/codeine 10mg prednisone 3mg Pro gel 1 dt 20. Can codeine phosphate also be used in the place of codeine? If so, in what quantity?

Not strictly speaking. In any case, it has to be converted. The amount of crystal water   must be taken into account, if present. It is also useful to ask the supplier of the raw material if he has an idea of the amount of water the powder might contain. The OMR account can be done on the basis of the molecular masses ofwwel based on the percentage codeine present in codeine phosphate. Such data can be found either in the Merck Index or the Martindale.