Answered by John Hsiao, M.D.:
Since both the newer and older anti-psychotic drugs cause sedation, they often have some immediate calming effects when started in someone with psychosis. The anti-psychotic effects, per se -- reduction of hallucinations and delusions -- may be seen in the first few days, but often don't become apparent until someone has taken the drug for a week or two. Many of the side effects associated with a drug (e.g., sedation, dry mouth, dizziness) become apparent immediately, and if anything, diminish with time. It may take many months for the anti-psychotic effects to develop fully, and sometimes, hallucinations and delusions merely decrease in frequency and intensity, without disappearing entirely.
Some evidence indicates that that anti-psychotic medications act to eliminate symptoms most quickly after a first episode of psychosis but that with repeated episodes it may take longer for the medications to show a full effect. This is one of the reasons why it is important to minimize the number of episodes or relapses a patient experiences. Over the short run, whether or not an individual adheres to the medication regimen prescribed by their doctor is the major determinant of whether or not they will experience an illness relapse.
Weight gain may be apparent from early on, but since it is generally gradual, it may take many months or years to become fully apparent. Weight gain might be a factor in switching from one atypical anti-psychotic to another or to one of the older anti-psychotics. The older anti-psychotics also caused weight gain, but not as often as many of the newer agents.