In reply to:
then one day i smoke some weed while on a couple medications, i never smoke weed the same way again, and i think the combination 'altered my brain chemistry

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strong psycological experience can "color" the way you think in similar,future instances. If you had a really bad experience mixing weed and medications,then future instances when you smoke weed it might remind you of the bad time you had on the mix (whether subconsiously or not) and trigger old feelings. I think this is called "psycological conditioning"

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This is basicaly a flashbacks, although it dosent have to happen when using drugs, it could be your listening to some music on the radio when you have a car crash killing your family, listening toi that tune from then on would cause serious psychlogical problem.

The action of lsd does mimic the brains neurotransmitters seratonin (5-ht) which have specific receptor sites that they are able to lock into. Sorta like a lock and key action. When the lsd locks into this receptor site it then inhibits the serotonine receptors in the brain. These are usualy responsable for the regulation of the other serotonin  synapses which is the space between neurons. So when this happens the previously regulated neurons are allowed fire off much faster.

Because it is the serotonin receptors are what regulate your perception increasing there hyperactivity causes an altered state. So basicaly they over clock your brain so you better get a good heat sink. . There is more than one type of seratonine but most psychedelics are usualy specific to only affect the 5-ht sites.