I Am Asking These Questions For My Son-in-law. He Is

I am asking these questions for my son-in-law. He is 37 years old and is on metoprolol succinate for high blood pressure but lately his heart has been racing and he is losing periods of time. His blood pressure is doing great on the medication but this series of events have been happening since they upped the dosage (about 2 weeks ago). He can't take anything else for his blood pressure because he is allergic to them so what should we think?