Can Long-Term Zinc Therapy Cause Foot Drop or Neuropathy in Wilson Disease?

Years of zinc-only therapy keeping your copper under control sounds like success — until your foot starts dragging. The uncomfortable truth is that foot drop and progressive motor neuropathy on long-term zinc monotherapy usually point to the opposite of what Wilson disease does: copper deficiency caused by zinc working too well. That said, residual damage from earlier copper excess and, in rare cases, zinc’s own neurological effects can also be in play. Separating these three possibilities changes treatment completely, so it is worth understanding each one.

How zinc can deplete copper to dangerous levels

Zinc works by blocking copper absorption in your gut. It triggers intestinal cells to make a protein called metallothionein, which traps copper and carries it out of the body in shed intestinal cells rather than letting it cross into your bloodstream.¹ This is exactly the mechanism that makes zinc effective maintenance therapy. The problem is that the margin between “enough suppression” and “too much suppression” is narrower than many patients and even some physicians realize.

When serum copper and ceruloplasmin fall below the normal lower limit — not just within range, but below it — nerve and bone marrow function can be compromised. Clinically, zinc-induced copper deficiency most often looks like a slowly progressive myeloneuropathy: sensory symptoms in the feet and legs, weakness, and in more severe cases the foot drop that is characteristic of peroneal nerve or corticospinal tract dysfunction.²³ A 2023 systematic review confirmed that this pattern of copper-deficiency neuropathy is a well-recognized iatrogenic complication of zinc overtreatment in Wilson disease.⁴

The key clinical clue is trajectory: if your neurological symptoms were stable or improving and then began to worsen after you had been well-controlled on zinc for years, copper deficiency from overtreatment is the most likely explanation. This is different from the worsening that can happen in the first weeks after starting a new chelator (a phenomenon called paradoxical worsening, discussed below).

Residual copper-excess damage: the legacy injury

Not all neurological problems that appear during treatment are caused by treatment. Some patients have had copper accumulating in their brain for years — sometimes decades — before diagnosis.⁵ Neurons lost to copper toxicity do not regenerate cleanly, and some functional deficits persist or even become more apparent as the acute inflammation settles.

If your foot drop appeared early in your treatment course — or was present before treatment started — and has remained stable rather than progressing, it may represent residual damage from the original copper loading rather than a new treatment complication. In this scenario, your copper indices (serum copper, ceruloplasmin, 24-hour urine copper) would be in normal range or at the expected low-normal for treated Wilson disease, and there would be no trend toward further depletion.

Distinguishing residual damage from active overtreatment requires watching the direction of change over time, not just a single measurement.

Zinc’s own neurological effects: a smaller piece of the picture

Zinc itself has been reported to cause neurological symptoms in a very small number of cases, independent of its effect on copper. The proposed mechanism involves zinc interfering with neuronal signalling directly.¹ However, this is rare and far less well-documented than zinc-induced copper deficiency. Before attributing neuropathy to zinc toxicity per se, your team will want to rule out overtreatment first.

What your doctor will look at

When you bring this concern to your specialist, expect them to review:

• Serum copper and ceruloplasmin — values below the lower limit of normal (not just “low for Wilson disease”) are a red flag for overtreatment.⁴
• 24-hour urine copper — persistently very low values may indicate over-suppression.
• Nerve conduction studies / EMG — to characterise the pattern of neuropathy (motor, sensory, mixed; axonal vs. demyelinating).
• MRI of brain and spinal cord — to look for signal changes in corticospinal tracts consistent with myelopathy.
• Treatment history and dose — particularly whether zinc dose was ever adjusted downward as your copper stores declined over the years of treatment.

The AASLD 2022 Practice Guidance notes that copper levels should be monitored regularly and that dose adjustments may be needed over time to avoid both under- and over-treatment.⁶

What treatment adjustment might look like

If overtreatment is confirmed, the approach typically involves reducing the zinc dose, temporarily adding a small amount of dietary copper supplementation, or switching to a different maintenance strategy — all under close specialist supervision. Recovery from zinc-induced copper-deficiency neuropathy can be slow (months) and is not always complete, which is why preventing it through regular monitoring is so important.³

If the problem is residual damage from past copper excess, neurological rehabilitation, physiotherapy, and assistive devices for foot drop (ankle-foot orthoses) can help manage function even without a pharmacological fix.

If you are currently on zinc monotherapy and have not had copper indices checked recently, this is a good reason to ask your specialist to review them at your next appointment. You can also read more about the general framework for maintenance decisions at /post/after-initial-chelation-can-i-switch-to-zinc-monotherapy-for and about the neurological side of Wilson disease at /post/early-symptoms.

Practical summary

Cause	Timing clue	Copper indices	Direction
Zinc-induced copper deficiency	Worsening after stable period	Below normal	Progressive
Residual copper-excess damage	Early in treatment or pre-treatment	Normal low range	Stable
Direct zinc neurotoxicity	Variable	May be normal	Rare, variable

This page is patient education, not medical advice. Foot drop and progressive neuropathy always warrant prompt evaluation. Please discuss your specific symptoms and copper monitoring results with your Wilson disease specialist before making any changes to treatment.

References