Pills imprinted with the name Xanax that are being sold on the streets of SF have now been blamed for the deaths of three people, and there may be more to come. We mentioned the first death on Friday, and since then two more people have succumbed to complications from opioid overdose, stemming from the pills which are laced with the drug fentanyl, as KRON 4 reports.

Fentanyl is a rapid-onset, short-duration painkiller that is used in hospital settings, sometimes in combination with benzodiazepines like generic Xanax or Ativan. As the CDC explains, fentanyl is 80 times more potent than morphine, and hundreds of times more potent than heroin, thus the danger of taking unregulated doses via a pill off the street. Fentanyl was used in aerosol form as an incapacitating agent by the Russian military in 2002 against terrorists corralled in a Moscow theater, and 127 of the hostages died.

The source of the pills in SF remains unknown, however just two days ago the CDC issued a warning about the rise of illegal fentanyl seizures around the country in the past two years. Also, they report, illicitly manufactured fentanyl can often be mixed with heroin or cocaine in order to enhance its effects.

Although pharmaceutical fentanyl can be diverted for misuse, most cases of fentanyl-related morbidity and mortality have been linked to illicitly manufactured fentanyl and fentanyl analogues, known as nonpharmaceutical fentanyl (NPF).

Nationwide there have been more than 700 reported fentanyl-related deaths since late 2013, though that number is likely much higher due to the fact that coroners do not necessarily test for the drug or its analogues.

The pills in SF are clearly labeled XANAX, however fentanyl mixed with other things also gets sold under the name "China White."

The three people who died in San Francisco were between the ages of 20 and 40, and as CBS news reports, there was a fourth person in the same home who was also hospitalized.

One of the dead has been identified as 34-year-old Andrea Choye, and there's some implication that the four, who apparently knew each other, may have been trying to "come down" after a night of partying on other drugs, though that's not confirmed.

Meanwhile, last week, CBS 5 reported that seven teens at Pinole Valley High School in the East Bay were caught taking what they said was Xanax and proceeding to act strangely at school. Five were hospitalized  something that really shouldn't happen with real Xanax?  and two were taken home by their parents.

Dr. Phillip Coffin of SF's public health department warns that the counterfeit Xanax pills are virtually indistinguishable from real, brand-name Xanax, as shown in the illustration above.

Related: Marin Kids End Up In Hospital After 'Triple C' Overdose