Beta-blockers can cause blood levels of potassium to increase.  Potassium is very healthy in normal amounts, but dangerous when too much is consumed.  Do not take any supplements containing potassium while on beta-blockers, and get your potassium levels checked regularly.  

Eat the following high potassium foods only in moderation:

bananas, citrus juices, avocados, cantaloupe, tomatoes, potatoes, lima beans, flounder, salmon, cod, chicken, and meat.