Channel blockers are chemical substances, ranging from ions to complex organic molecules, that bind inside the pore of an ion channel and block the flow of ions through that channel. A subset of channel blockers, known as "open channel blockers" have access to their intra-channel binding site only when the channel is in the open configuration (i.e. in the configuration that conducts transmembrane ion flux). Open channel block is characterized by "flickery closings" in single-channel recordings.

Some examples of:

Channel blockers

• Tetraethylammonium (TEA)-- several voltage-dependent potassium channels
• Caesium -- several voltage-dependent potassium channels and "H-type" channels

Open channel blockers

• Magnesium -- NMDA receptor channel
• Cadmium -- voltage-dependent calcium channels
• Phencyclidine (PCP, angel dust) -- NMDA receptor channel