So we have a very complex system, highly tuned over the past millenia do we? Wrong. We don't. The human metabolic pathways are riddled with fundamental, stupid errors that are far from perfect. Biochemists have pointed out the errors over many years.
Stuffing foreign chemicals? What are foreign chemicals? Is drittidochlorobenzyhexhidrine, 6-7, diflycaramentain a foreign enough sounding chemical to you? I've got news for you - it's a squirt of lemon juice in a bottle of water. Don't be too certain about what you regard a forign chemicals.
Prescribed medicines don't target intended targets. They are all composed of molecules no matter how complex you may think they are and are metabolised by the liver. The liver may alter their chemical composition in order to perform the desired function and they are only effective after the liver has worked on them ( with a few exceptions). Without the intervention of the liver, most drugs/medications would be useless.
What are intended targets? You go to a doctor with bronchitis and he prescribes amoxycillin for you. Do you really believe that the drug will only target your chest infection and not the other sensitive bacteria that may be present in your ear canal (causing otitis media) for example or other parts of your body. Define targeting.
You really need a better understanding of this basic human biochemistry/pharmacokinetics and revise your definition of foreign chemicals before you post such a puerile question on The Answerbank if you seek a truly definitive answer.