Chemistry - Why are drugs mixed with their sodium salts?

A large percentage of pharmaceuticals are sold as salts, and sometimes even as co-crystals, i.e. a drug cation and a drug anion. You are right in the sense that the basic purpose is to make them soluble but this is not the only one. If it is a intravenous drug, salt form may have less pain. A low molecular weight medicine might be in a liquid form yet its salt is a solid. Sometimes you want to make a hydrophobic salt so that its stability is higher in high humidity. Sometimes you wish to improve the taste. There are plenty of factors.

Most likely the reason for a mixture of medicine and its sodium salt is its controlled release and some buffering action. In case of your carmicide which is a mixture of sodium citrate and citric acid, is a buffering system and secondly citric acid alone is very corrosive and very bad tasting (tart). Fucidin is only allowed as a Na-salt.

This review will tell you the principles behind choosing a salt.

Go to Google Scholar and read: Gupta et al. Salts of Therapeutic Agents: Chemical, Physicochemical, and Biological Considerations, Molecules 2018, 23, 1719; doi:10.3390/molecules23071719

The choice of a particular salt formulation is based on numerous factors such as API chemistry, intended dosage form, pharmacokinetics, and pharmacodynamics. The appropriate salt can improve the overall therapeutic and pharmaceutical effects of an API. However, the incorrect salt form can have the opposite effect, and can be quite detrimental for overall drug development.

API= active pharmaceutical ingredient