Drugs can work to prevent axons from firing action potentials which prevents synaptic transmission. This can occur in many ways.

Drugs preventing synaptic transmission 

Some drugs are able to alter synaptic transmitter release. This can be done through many ways but include the over stimulation and/or prevention of the release of transmitters. Some are even able to target specific transmitters. 

Drugs alteration of synaptic transmitter release

One way drugs can effect presynaptic transmitter release is through modifying the systems that the neuron uses to monitor and regulate its own transmitter release. This includes altering the auto-receptor that monitors how much transmitter has been release presynaptically. 

Drugs altering transmitter release through modulation of presynaptic activity

Even with an adequate supply of transmitters in vesicles in the presynaptic terminal, various agents or conditions can prevent the release if transmitters when an action potential reaches the terminal. Examples of this include: 
- The prevention of synaptic transmission 
- Alteration of synaptic transmitter release
- Alteration of transmitter release through modulation of presynaptic activity 

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One way drugs can change synaptic transmission is by modifying the behavior of the presynaptic neuron, changing the system that converts an electrical signal (an action potential) into a chemical signal (secretion of neurotransmitter). Most common presynaptic drug effects are: 
- effects on transmitter production 
- effects on transmitter release 
- effects on transmitter clearance

Drugs altering presynaptic events

Breedlove, S. & Watson, N. (2017). Behavioral neuroscience, eighth edition. Oxford University Press.

Behavioral Neuroscience 8th Edition

For a presynaptic neuron to produce neurotransmitters, the axon terminal must receive a steady supply of raw materials and the enzymes that use them to synthesize transmitter molecules, vesicles, and other important components. Some drugs alter this process by: 
- inhibition of transmitter synthesis
- blockade of azonal transport 
- interference with the storage of transmitters

Drugs affects on presynaptic transmitter production

Drugs effects presynaptically on transmitter release

Immediately after release, transmitter is rapidly cleared from the synapse. Getting rid of the used transmitter is normally an important step because unit it is gone, new release of transmitter from the presynaptic side won't be able to have much extra effect. 
 This can occur through: 
- The inactivation of transmitter reuptake
- The blockade of transmitter degradation

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