I am on medication for my blood pressure. What over-the-counter medicines can I use for a respiratory infection?

For practically all forms of heart disease, but in particular hypertension and its treatment, the single most important over-the-counter medication to avoid is a “decongestant”. All decongestants are drugs which cause constriction of blood vessels, which obviously would not be desirable. If you have any questions, consult with your pharmacist and he can prescribe a cold remedy which would be appropriate for your situation.