Several factors. Doctors select appropriate medication based on available evidence. We take into account any other conditions you may have that would lead us in one direction over another. Example, diabetics should be on ace inhibitors. Persons with a history of heart attacks should be on certain meds as well. Other meds are to be avoided in certain instances. Familiarity of the doc with meds is important too.
Depends on disease. Certain diseases require a particular class of BP meds to be used first. Diabetes usually requires a class called ace inhibitors or arbs if there has been problems with ace in past. People who have had heart disease require beta blockers such as metoprolol first. Others begin with diuretics in many cases. Personal factors, such as allergies, cost and side effects can also affect choice.