Is it OK to use "Group A, B, C ..." in a manuscript?,
In general, on the biomedical field Latin letters (A/B/C or a/b/c) are reserved for the figure panels (e.g. "Figure 1 A and B clearly show..."). Typically, biomed articles tend to have a lot of panels per figure, so it is suboptimal to use these letters for anything else. It is much more handy for the reader, if experimental groups have human-readable names that are clearly defined in the method section. A little fantasy may be helpful. For example,


Group 1 - placebo control - "Placebo" 
Group 2 - 10 mg Blabla drug - "B10"
Group 3 - 25 mg Lalala drug - "L25"
Group 4 - Blabla drug + Lalala drug, once per day - "BLI"
Group 5 - 20 mg Blabla + 30 mg Lalala - "B20L30 and etc.