Heroin use among people who also use fentanyl appears to have seen a "dramatic" rise in the span of just a few months, particularly in the western U.S., according to new data from specialty lab Millennium Health provided first to Axios.

Why it matters: The data suggests a rapid reversal in drug use trends and reflects a shift amid reports that fentanyl is currently in short supply in the U.S., said Eric Dawson, Millenium's vice president of clinical affairs.