How does heroin effect the heart?

It depends on which drug you are thinking about!

There are many legal drugs that affect the heart. Some slow the rate and help lower the blood pressure. Others regulate the heartbeat. Some drugs speed up the heart rate. There are even drugs that paralyze the heart for open-heart surgeries.

Some legal drugs also have side effects on the heart and have to be taken carefully.

Illegal drugs are illegal because they're posions, so they're the most harmful to the heart and the rest of the body. Most of them either cause the heart to race uncontrollably and increase the blood pressure to dangerous levels, or relax the heart so much that it doesn't beat normally.