Chances are that the rash in their mouth is exactly the same rash that they have on their skin.
I can refer them to a dermatologist and in consultation with the specialist, we can come up with a diagnosis and treatment to help the patient. The mouth is connected to the rest of the body. These drugs are known to cause stomach ulcers and they can also cause oral ulcers. The dentist has to consider what else is happening to the patient that might be affecting the person's mouth. There are many systemic causes for oral conditions.