I came across a male patient with these odd purpuric lesions which where pigmented on his legs. This was 3 months after starting Metformin for diabetes. Not too sure what was going on I sent him to see our dermatologist and a diagnosis of Pigmented Purpuric Dermatitis was made.
In essence it is a capillaritis with T cell inflitration of unknown origin. If the medication is suspected stopping it for many months is recommended. They can dissapear in time or just hang around for many years. Steroids are not useful except for any itching. They look like this (courtesy of Derm Net) and you can read more about it here.