There are lots of drugs that are synthetic and have pretty much the same effect as illegal drugs.
A lot of the addictive nature of feel good drugs is that they give pleasure, so people do them more and more often because they like the feeling they get from them. There is no specific chemical that makes a drug addictive, it is the combination of all then chemicals in the drug that make people feel good and therefore make people want to do them.
However there is also a physical addiction, which means that people actually start to have adverse affects if they do not take the drug, sweating and shaking for instance so I suppose drugs could be made such that they do not create that physical addiction but in any case, if a person stops taking drugs, the physical side of things mostly wears off after a couple of weeks but then the bigger problem to overcome is the fact that people still want to get the feeling they got on the drug, so end up doing it again.