Worth a read ,
1. The Leg Heaviness, Spasms, and Electric Shocks (The Somatic Nervous System)
You asked about the major clusters and biggest nerves in the body.
• The Sciatic Nerve: This is the largest and longest single nerve in the human body. It runs from your lower back, down through your hips and buttocks, and all the way down each leg. It controls the large muscle groups of your thighs and calves and carries sensory signals from the legs back to your brain.
• The Spinal Cord Clusters: The spinal cord serves as the main electrical superhighway. Motor neurons cluster heavily in the lumbar (lower) spinal cord to control leg movement and gait.
Why it is affected:
Mirtazapine works by significantly altering norepinephrine and serotonin, and at low doses (like 7.5mg), it acts as an incredibly potent antihistamine. When you stop the medication, your nervous system experiences a rebound effect. The sudden drop in antihistamine and noradrenergic stabilization causes hyperexcitability along your largest nerve paths.
The "electric shocks" (often called brain zaps or body zaps) are caused by transient, chaotic firing in your sensory nerve networks as they try to relearn how to signal without the drug. The profound heaviness and spasms happen because the motor clusters in your spinal cord and the sciatic nerve path are temporarily misfiring, leading to a feeling of uncoordinated, weighted resistance in your leg muscles.
2. The GI Tract Distress (The Enteric Nervous System)
Aside from the brain and spinal cord, the largest cluster of neurons in the entire human body is in your gut. This is called the Enteric Nervous System (ENS), often referred to by scientists as the "second brain." It contains over 100 million neurons embedded directly in the walls of your gastrointestinal tract.
Why it is affected:
The gut relies heavily on the exact same neurotransmitters that mirtazapine alters—especially serotonin and histamine. In fact, mirtazapine specifically blocks a receptor called 5-HT3, which is heavily concentrated in the gut and directly controls nausea, motility, and digestion.
When you withdraw from mirtazapine, the 5-HT3 and histamine receptors in your enteric nervous system experience a massive, sudden destabilization. Because the gut has its own massive, independent "brain" of nerve clusters, this receptor chaos causes immediate physical fallout: altered gut motility, severe nausea, cramping, and a hyper-reactive GI tract.
What is Happening Globally: Protracted Unbalance
What you are experiencing after 4 months of use is a protracted state of neurochemical unbalance. Your body did not just experience a "chemical clearance" of the drug (mirtazapine leaves the blood within days); rather, your nerve cells physically changed their architecture while you were on it. They altered their receptor densities to accommodate the drug. Now that it is gone, those structural changes require a significant amount of time to revert to their original baseline
0mg — 4 months — completed taper
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