Now, a photo of Cassandra is featured on a giant billboard that Saldivar is renting near AT&T Stadium in Arlington, Texas, to try to raise awareness of the dangers of fentanyl.
"[Fentanyl deaths] are becoming more and more common and I don't want anyone to go through the same thing that my family is going through," Saldivar told "Good Morning America." "I wanted to do it to make people aware to not be taking anything that is not prescribed to them."
Saldivar said she received a phone call in the middle of the night on June 1 that Cassandra, the mother of a 2-year-old son, was in the hospital.
Cassandra Saldivar, who passed away on June 1, 2021, is pictured with her son. (Patricia Saldivar)
When she arrived, she learned that her youngest child had passed away.
"It was a complete shock," said Saldivar, who said she was told Cassandra had taken a pain medication that was laced with fentanyl. "I was like what in the world. I didn't know it was possible to lace a medication with fentanyl."
Patricia Salvidar is photographed with her late daughter, Cassandra Saldivar. (Patricia Saldivar)
The billboard that now prominently features Cassandra's photo reads, in part, "1 pill that's all it took ... fentanyl kills."
Saldivar, who is also helping to raise awareness by speaking out on TikTok and handing out flyers to high schools, said she wishes she had known as a parent about the dangers and increasing prominence of fentanyl in the United States.
"If I had known this before, I would have warned my kids, I would have said, 'Watch it. You better be careful,'" she said. "I've been getting a lot of thank you's from people saying that they're going to talk to their children."
Patricia Saldivar rented a billboard in Arlington, Texas, in memory of her daughter, Cassandra. (Patricia Saldivar)
Fentanyl is the primary driver of the significant increases in drug overdose deaths in recent years. More than 93,000 people died of a drug overdose last year, according to the Centers for Disease Control and Prevention.
Last month, top law enforcement officials in the U.S. announced the seizure of more than 1.8 million counterfeit pills during a coordinated series of law enforcement raids throughout the country since early August.
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The amount of fentanyl-laced pills seized by the Drug Enforcement Administration over the past eight weeks is enough to kill 700,000 people, DEA administrator Anne Milgram said at a Sept. 30 press conference.
"We cannot stress enough the danger of these counterfeit pills," Milgram said. "We're seeing these pills being illegally sold in every state in the United States. They are cheap, they are widely available, they can be purchased online and on social media -- so through people's phones, and they're extremely dangerous."
The pills are often made to resemble real prescription opioid medication like Oxycontin, Vicodin, and Xanax, or stimulants like Adderall, according to the DEA. Most are made in Mexico, with China supplying the chemicals.
The DEA made clear in its announcement that this epidemic of fake pills does not impact legitimate prescriptions filled at a pharmacy. Only those sold on the black market.
Patricia Salvidar, far right, poses with her late daughter Cassandra and loved ones. (Patricia Saldivar)
"Do not take anything that is not prescribed to you by a doctor, even if it's from a friend," said Saldivar of her advice to others. "If the bottle does not say your name on it, don't take it."
She added that her wish is that by speaking out about Cassandra's death, she can help save others' lives.
"It helps with my grief because I know I'm helping others," said Saldivar. "Cassandra's passing was not in vain. I'm keeping her memory alive."
ABC News' Quinn Owen & Alexander Mallin contributed to this report