However, its not as simple as too much of one thing or too little of another. An excess of dopamine in the brain has been linked to schizophrenia, yet not all schizophrenics who are put on drugs that block the action of dopamine have been cured. Similarly, decreased serotonin and norepinephrine have been linked to depression. Some people find that drugs that effect both serotonin and norepinephrine are useful in treating depression, others find drugs that only act on serotonin to work, while still others have yet to find a drug that works for them. In the cases where drug treatments aren't effective more complicated interactions between different chemicals and parts of the brain must occurring, or an entirely different chemical imbalance may be causing the disorder, thus the drug is ineffective.
Another confusing aspect to drug therapy is that it has been shown that the drugs begin working within several hours (blocking the action of dopamine receptors or blocking the uptake of norepinephrine and serotonin) but the person taking the drugs does not feel better right away. It takes several weeks before the symptoms of the disorder are alleviated. This suggests that the psychological symptoms (delusions, feelings of worthlessness etc.) are more complicated than an imbalance of chemicals.