The Biology of Addiction -


What little is already known about the biochemistry of addiction is merely a discussion of the possible interactions of neurotransmitters with the mesolimbic dopamine system where pleasure is produced and how some of the addicting substances produced by plants are similar to, and imitate, various neurotransmitters (1,2,3,4).   The literature seems to contain very little about how addiction could enhance survival (5).


 My work as a physician for 60 years has intermittently confronted me with various addictions including caffeine addiction in myself (see below).  Also evolutionary biology has been an intellectual hobby of mine since high school which was much help in understanding what I learned in medical school and observed in medical practice.  From time to time I have wondered how could addiction have come about? My current hypothesis follows.


Tolerance is a mechanism permitting consumption of plants with toxic effects on the brain.  Tolerance is what causes drug addicts to be able to survive, even thrive, on otherwise fatal doses of the drug to which they are addicted.  A corollary to this is the idea that addiction has persisted by being tied incidentally, but inextricably, to tolerance.  Thus, seemingly harmful traits can survive natural selection if they are associated with a trait of great benefit to the species.  Could drug addiction be such a harmful trait unavoidably connected with tolerance and could its survival value be in permitting greater consumption of a “dangerous” food?  Could addiction be a vestige of an inheritance from some ancient, long extinct ancestor who actually benefited from the trait (most likely a fish)?  There are modern insects whose larvae become “addicted” to toxic plants to render the entire species toxic to predators, for example: butterflies of the genus Parides (6).


So the following scenario may partially explain the origin of addiction.   Unlike the Parides larvae which hatched on a poisonous Aristolochia leaf, our aquatic ancestor (“most likely a fish”) which “invented” addiction would have to keep seeking the poisonous food that rendered its lineage poisonous and therefore less subject to predation.  Addiction served this purpose by overcoming the temptation of ample alternative food choices.  In our case, the ability to become addicted just happens to have persisted after its benefit became moot.  Compare with our appendix persisting also with negative benefit.  More toxic animals and plants inhabit the marine environment than our terrestrial one.  Undoubtedly some of the toxic species utilize second hand toxins as the Parides larvae do—PhD thesis material awaiting elucidation along with how were/are the fish hatchlings induced to eat enough of the toxic food to become addicted.  If we can find a poisonous marine vertebrate that achieves toxicity by diet, this will begin to confirm the hypothesis of an evolutionary origin of addiction especially if that species bears live young so that they would be born addicted as are human infants born to mothers who are currently addicted to heroin.  Experiments with a captive population of such animals should easily demonstrate tolerance or even tachyphylaxis, another universal feature of the addiction syndrome (along with tolerance and drug-seeking behavior).  An explanation of tachyphylaxis follows.

Here is my personal experience with caffeine addiction—significant because it was so extreme but still not disabling or otherwise harmful except for occasional withdrawal headaches.  When I lived in Afghanistan from 1968 to 70, I frequently drank tea in order to get boiled water without insulting the local water.  Actually I dislike tea and frequently easily weaned myself from it in about a week unless I was expecting to travel.  It takes much tea to fill one’s water requirement in a tropical environment, so my addiction exceeded toxic doses at times. Tachyphylaxis restored my addiction sufficiently promptly to avoid any toxic symptoms, emphasizing to me how tachyphlaxis got into the addiction package along with a withdrawal syndrome and tolerance.


My unusual personal insights into tachyphylaxis came in the next decade or so. The amazing discovery was that on the day after a few midday coffees, I would get a headache 4-5 hours after the first “missed dose”.  I have had no experience with patients having such headaches if they drank less than 5-6 cups per day. Now I drink no more than a cup or two of real coffee a month.


The bottom line: regard addiction as like having an appendix and don’t bother with the cure until threatened by some inconvenience.



1)      Mahler Steve. The nature of addiction, Evolution and Psychopathology-12/1/04.  Available at:                                    nature_of_addiction.doc  accessed July 4, 2008                

2)      Umanof DF. Addiction: an unintended consequence of evolution.  Avalable at:

     accessed August 3, 2008

3)      Berridge KC, Robinson, TE. What is the role of dopamine in reward: hedonic impact,                reward learning, or incentive salience? Brain res, Brain res reviews. 1998 Dec;28 (3);309-69

4)      Nesse, RM, Berridge, KC, Psychoactive drug use in evolutionary perspective.  Science. 1997 Oct 3; 278 (5335): 63-6

5)   Nesse, RM. Evolution and Addiction (commentary) Addiction. 2002: 97(4):470-1

6)   Young , Allan. 1991. Sarapiqui. Smithsonian Inst. Press 1991

       For more references and more about the appropriateness of evolutionary biology in our medical curricula please refer to my website:   Look near the bottom of the category “Other Science and Technology” for Biology’s Integrating Insights for Medical Science.

John A. Frantz, July 28, 2008,                         email:

812 22nd Avenue                                                              telephone:  608 325 3242  

Monroe, WI, 53566-1672