Though scientists try and make the drugs as specific as possible, they are only partially successful. The drug has a powerful effect on the heart and a lesser effect everywhere else. At the same time, drugs are continually elminated from the body through the kidneys' producing urine, and thorugh the liver by chemical breakdown. The healthier these waste-disposal systems, the less damage is done to the body by drugs.
All drugs are poisons to a greater or lesser extent.
There is no such thing as a completely harmless drug. But whether or not you will get a noticeable side effectds depnds on dosage, kidney and liver function, constitution, general health and resistance, age, diet, and many other factors. Doctors can never predict who will get which side effect. It is also a question of sensitivity. A healthy person may feel a side effect which is not felt by his unhealthy cousin.