When you take medication for problem how does it work? How does the medication know how to go to ares of your body.
Some one said some thing about receptors and agonists what do they do?
Why do some people get side effects from taking medication?
Why can the human body produce its own medicine with out side effects!
That's a great set of questions. Unfortunately, the answers can be so complex that even someone who spent 6-8 years earning a doctorate in the field can only give you incomplete answers. I spent a good chunk of my career doing medical research, so I'll try to give you some general answers.
Most medications work by floating around the body long enough to come across their "targets" by chance. A lot of the work that researchers do is to try to find ways to increase the odds of a medication finding its target. Targets are usually proteins, but sometimes they can be RNAs or other kinds of molecules. There are thousands of potential drug targets, but the number of targets for which we actually have medications is less than a thousand.
When a medication finds its target, it usually will stick to it or "bind" to it. One way to think of this is as locks and keys. In this case, the lock is the target, and the key is the medication. Usually, targets naturally bind to other molecules in the body to carry out their function. For example, there is a whole class of proteins called "kinases" that are involved with normal functions, but also with many cancers. When something binds to a kinase in just the right place and in just the right way, the kinase turns on or becomes "activated". Many types of cancer occur because one or more kinases have become too active, especially when those kinases are involved in "pathways" that cause cells to divide or grow.
This is where things start to get interesting...and complicated. Many medical conditions involve networks of dozens or more proteins turning each other on or off in complicated "signaling pathways". Let's say that we have a favorite kind of kinase. It sits on the surface of a particular kind of cell, but part of it actually sticks inside of the cell. If you do something to it on the outside of the cell, it can change its shape inside the cell, which signals to the cell that something important is happening. Proteins that do this are called "receptors".
When something sticks to our friend, the "receptor kinase", in just the right way outside of the cell, it gets activated (turned on), and the part of it that is inside the cell tells some proteins to turn on and others to turn off. Each of those proteins interact with more proteins to carry out all sorts of activities in the cell. If something goes wrong with our receptor kinase, and it stays turned on, the proteins "downstream" in its signaling pathway might tell a cell to keep dividing and growing until you have cancer.
How might a medication act to fix this? Luckily, there are lots of ways a medication might work. Maybe you could find a drug that binds to the receptor outside of the cell and turns the receptor off. That's called an "antagonist". That's great, and it might fix the cancer. The problem is that it also knocks out all of the normal signaling that our favorite receptor kinase does, so the medication might cause other medical problems. This is one type of side-effect.
Another thing you could do is try to find a medication that gets inside the cell and binds to the part of the receptor kinase inside the cell that actually activates and deactivates other proteins, i.e., the "business end" of the kinase. It's a good idea, and sometimes it's all we have. The problem with this is that in the case of kinases, this part of the kinase protein is almost identical across the hundreds of different kinases we have in our bodies, so unless we can target our medication very, very specifically to the kinase we want, our medication will stick to nearly all of the body's kinases at once, blocking their activity and leading to side effects all over the body.
A better approach probably is to try to find a medication that can stick to one of the proteins "downstream" in the signaling pathway, since it may be more specific in its impact than the original receptor kinase. If a downstream protein tells a cell to divide, you would try to find an antagonist to turn it off. If a downstream protein tells a cell to stop dividing, you would try to find an "agonist" medication that activates that protein. (As a side note, I'll mention here that most drugs are antagonists, i.e., things that block activity, since it is much easier to break things than it is to build things.)
As you can imagine, depending on which part of the pathway you target, you may be too blunt in your approach or not blunt enough. It's rare to get it perfect.
A little more about side effects: nearly every medication has potential side effects. By the way, that goes for natural supplements and even a lot of the food that we eat. Another kind of side effect happens when there are completely unrelated molecules in your body that have shapes similar to your target, and you might not even be aware of them. The problem here is that your medication might stick to these other molecules inadvertently, causing essentially unpredictable side effects. This is called an "off-target" side effect.
I've written a lot, but I've barely scratched the surface. Signaling pathways can get frighteningly complex in a hurry. That's one reason why the same drug can have very different effects on different people. For example, I share around half my genes with my sister, but if we take the same prescription cold medicine, I'll fall asleep for twelve hours within fifteen minutes, while she'll be bouncing off the walls as if she had finished a whole pot of coffee by herself.
To your final question, "Why can the human body produce its own medicine without side effects?", that's just not true. Ask anyone who has an autoimmune disorder.
