Who regulates what
Which board you file with depends on who was involved. For a physician — or for anyone you believe was practicing medicine without a license (like an aesthetician injecting) — that's the Medical Board of California (mbc.ca.gov). For a registered nurse or nurse practitioner, it's the Board of Registered Nursing. For a physician assistant, the Physician Assistant Board. For an aesthetician acting outside their scope, the Board of Barbering and Cosmetology.
If you're not sure which one, start at the Medical Board — and all of these boards are reachable through California's Department of Consumer Affairs at dca.ca.gov.
What to include
Give them the specifics: the business name and address, the date, the name of the person who treated you, exactly what happened, and any photos or paperwork you have. The more concrete you are, the more they can do with it.
Why it's worth the ten minutes
Filing feels like a hassle, especially if you weren't seriously hurt. But these boards mostly act on patterns, and your complaint might be the second or third one that finally gets a dangerous operator looked at. The person you protect is usually the next one in the chair.
See the spas that passed the public-record check