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The scientific explanation is that the needles inserted at appropriate points stimulate the nervous system to release chemical and hormones that affect the body's internal regulating system.
This differs from person to person. For more chronic complex condition 1-2 weekly treatments spread over months may be required. Acute issues may require fewer visits. For health maintenance annual to quarterly visits may be all that is needed.
Acupuncture needles are made of stainless steel and are extremely thin. Most patients only feel mild discomfort while the needles are being inserted; many feel no pain at all.