Some psychoactive drugs mimic the effect of a neurotransmitter on the receptor sites of a postsynaptic neuron. In doing so, the drug can similarly affect the postsynaptic neuron.

Nicotine is an example of a psychoactive drug that acts in this manner. Nicotine mimics the effect of acetylcholine on the postsnynaptic receptors. Another example is morphine which acts on endorphin receptors. Marijuana is thought to mimic the effect of a newly suspected neurotransmitter, anandamide. 

In the scene below, the red circles represent the molecules of the drug.

You may view this agonist mechanism by clicking on the "Play" button in the scene below. To view the animation again, you must first click on the "Rewind" button.