A dental assistant supports the dentist, so some things stay off-limits almost everywhere no matter how much training you have.
Assistants don't diagnose disease, don't drill or extract teeth, don't give anesthetic injections, and don't perform the scaling of calculus that is the licensed hygienist's work — and they never practice unsupervised.
Those lines hold even in states with generous expanded functions, because they turn on diagnosis, surgery, and licensure, not on task difficulty.
The short answer
Most of what a dental assistant may do varies by state, but a few limits are near-universal.
An assistant can't diagnose a dental problem or write a treatment plan, can't perform irreversible or surgical procedures like drilling and extracting, can't administer anesthesia, and can't do the licensed scaling that treats gum disease.
And an assistant always works under a dentist's supervision — never independently.
The reason these hold everywhere is that they're defined by diagnosis, surgery, and licensure rather than by how hard the task is.
Expanded-function training can widen an assistant's scope a great deal, but it doesn't move any of these lines.
The expanded-function middle ground still varies by state
What only the dentist can do
The core of dentistry stays with the dentist.
Only a dentist can diagnose dental disease and decide on a treatment plan, prescribe medication, and perform the surgical and irreversible procedures — preparing (drilling) a tooth, removing decay, and extracting teeth.
Anesthesia sits here too: administering local anesthetic injections and any sedation or general anesthesia is the dentist's responsibility (local injections are also within a hygienist's scope in some states).
An assistant's role around these is to support — never to perform them.
Our guide on whether an assistant can administer anesthesia or sedation covers the one narrow exception (monitoring, not administering, nitrous in some states).
What belongs to the hygienist, not the assistant
One line trips people up constantly: the difference between polishing and scaling.
An assistant may, in many states, perform coronal polishing — polishing the smooth crown surfaces of the teeth.
But scaling and root planing — removing hardened calculus to treat gum disease — is the licensed dental hygienist's work, and it's off-limits to assistants.
So an assistant can help make teeth look and feel clean, but can't perform the clinical periodontal treatment a "deep cleaning" actually refers to.
That procedure requires the hygienist's license, full stop.
Where expanded functions stop
Even the most advanced expanded-function assistants run into the same wall.
An EFDA in an authorizing state may place and finish a restoration — but only after the dentist has diagnosed the decay and prepared (drilled) the tooth.
The assistant never does the cutting or the diagnosing.
That's the pattern across every expanded function: the assistant may carry out a delegated, defined step, but the clinical judgment and the irreversible work stay with the dentist.
Expanded functions add to what an assistant can do, not to what they can decide.
Assistants always work under supervision
Finally, a dental assistant doesn't practice independently.
The role exists under a dentist's supervision by definition, so there's no equivalent of the hygienist's direct access — an assistant can't set up and treat patients on their own judgment.
Even a routine permitted task shows the pattern: an assistant may take x-rays with a state radiology permit, but the dentist interprets them and makes the diagnosis.
The support-versus-diagnose distinction runs through everything the role does — and it's the clearest way to understand what an assistant can't do.
This article is general career and scope-of-practice information, not legal advice. Dental assistant scope, permitted expanded functions, and their limits vary by state and change over time — confirm current requirements with your state dental board.