Many illegal drugs are dangerous, so there should definitely be penalties for using them. However, they should not be the same as they currently are. Many of these drugs are addictive, and we need to treat drug addiction as a mental disorder and give people help instead of just penalizing them.
Many illegal drugs should be decriminalized. Governments have failed at fighting illegal drug use and trafficking. In fact, the war on drugs has actually made society more violent and dangerous. Decriminalizing illegal drugs would mean that governments would spend less money trying to enforce broken drug laws. Also, crime and violence would decrease as well. In short, there should not be as many criminal penalties for drug use.
I believe the penalties for using illegal drugs is the harmful effects the drug has on your body. Further, I don't think it should be up to the government to tell people what they can and can not put inside their bodies. While there are governing bodies like the FDA and the USDA to help protect our food supply, if people want to put crap in their bodies, then so be it.
Years of experience around the world shows that the war on drugs simply doesn't work. It sends vast numbers if users, who for the most part are people with mental health issues to prison and criminalizes them. Look to Portugal where drugs have been decriminalized and drug usage has actually gone down. It also frees up police time to concentrate on catching the real bad guys - the dealers and organized crime gangs which control the drug trade.