It is difficult to make generalisations as there is a difference in the way a drug acts within a person's metabolism after it is absorbed into the bloodstream according to the age and weight of the person, as well as whether the drug is water or lipid-soluble. Once absorbed, most drugs do not spread evenly throughout the body.
Water-soluble drugs tend to stay within the blood and the fluid that surrounds cells. Lipid soluble drugs tend to be absorbed more quickly than water-soluble agents and therefore have more rapid effect. Lipid-soluble drugs may be stored in fat cells and continue to be released. Thus, when patients cease to take their medication the beneficial effects of the drugs may still be released for a while, which is obviously desirable.
A problem in treating mental illness with drugs has been the blood-brain barrier. Anxiety, panic and intrusive thoughts occur in the brain. The blood-brain barrier will allow small lipid-soluble molecules to freely move from blood to brain, and hence lipid-soluble drugs are found more effective.