How can a substance introduced into the blood so quickly have effects that happen hours later?

Answer 1

It is true that the quickest way to achieve effects of a medicine/drug is to introduce it in the bloodstream. Why bloodstream? Because, the blood will carry that chemical (we should call any medicine/drug a chemical) to every part of the body. Think how many times our heart contracts and retracts itself per minute? Average is 72 times/minute which we call heart rate.

Now let us talk about the mail questions: Some chemicals can bind to the proteins and lipids in our blood. These protein and lipid/fat bound chemicals can take longer to be processed by our body to finally expel form the body. Some chemicals because of their nature too can take longer for the body to process. The main organ that processes most of the medicines is the liver. Liver has so many enzymes to work for the medicines. These enzymatic activities make sure that the chemicals are processed in a way that the body can utilize as well as to make sure the toxic chemicals are converted into less toxic form. Liver has a capacity to convert medicine. so, sometimes it can take hours for liver to metabolize those chemicals (yes, processing those chemicals in body is called metabolism).