When Colorado legalized marijuana, it became a pioneer in creating new policies to deal with the drug.

Now the state’s surgeons, nurses and anesthesiologists are becoming pioneers of a different sort in understanding what weed may do to patients who go under the knife.

Their observations and initial research show that marijuana use may affect patients’ responses to anesthesia on the operating table — and, depending on the patient’s history of using the drug, either help or hinder their sym...