Do any health conditions or medications prevent a person from being certified to scuba dive?

Yes, as a part of the certification process, you are required to complete a health questionnaire, checking any past or present medical conditions.  If any medical conditions were checked, you will be required to have a physical examination performed by a doctor.  If the checked item is no longer a medical issue, you will be cleared to proceed with the certification process.  Otherwise, you may be prevented from the certification process.  The medical conditions list is quite lengthy and is available through PADI or NAUI but a few examples include being overweight, pregnant, taking prescription medications, age 45 and older, a current smoker, living with high blood pressure, or having a family history of heart disease, stroke, or high cholesterol.