[2] Excitation of the GABA receptor produces an influx of negatively charged chloride ions, which hyperpolarizes the neuron and makes it less likely to give rise to an action potential.
When these drugs are taken together, especially with ethanol (drinking alcohol), there is a disproportionate increase in toxicity because the effects of both occur simultaneously and add up since they act on the same receptor at different sites.
Having been on one or more antipsychotics for any appreciable amount of time results in dramatically reduced sensitivity to others with similar mechanisms of action.
MAO inhibitor drugs block an enzyme system resulting in increased stores of monoamine neurotransmitters.
Most of these drugs share a high affinity for the 5-HT2A receptor subtype, known to result in their common perceptual and psychological effects.
Findings of cross-tolerance with nicotine in animal models suggest that it is also possible in humans, and may explain why the two drugs are often used together.
[7] Cigarette smoking produces increased metabolic tolerance to caffeine due to upregulation of the CYP1A enzyme family (see aryl hydrocarbon receptor).