Diane Sawyer, ABC News
Next, tonight, heroin addiction in America. A stark number. The number of people using heroin here has doubled in five years.
700,000 people needing help tonight. And everyone who loved Philip Seymour Hoffman is looking back at the recent pictures of him for clues, for the warning signs. Would you know if it was someone you loved?
ABC's Cecilia Vega, now, with a very personal story, reaching across so many lives.
Reporter: It all seemed so normal. The last day of Philip Seymour Hoffman's life. Coffee in his New York neighborhood. Dinner with friends. But it would be his last because of the unrelenting pull from that demonic drug, heroin.
I did everything I could. Reporter: Those who have tried it say depression, anger, sadness, all gone in heroin's intense euphoria. Actor Russell brand described it like this--it transforms a tight, white fist into a gentle, brown wave.
The rush hits within seconds. After just 30 minutes, heroin can knock you out cold. There's a tendency to be just kind of going to sleep in the middle of a sentence.
Reporter: The high wears off after a few hours. You can be a functioning heroin addict. We see it all the time.
Reporter: To spot a hidden user, there are clues. Those heart-stopping three "Ps" of heroin.
Paraphernalia--look for spoons, needles, foil.
Personality changes, like increased irritability, theft.
And physical effects--track marks, small pupils.
I know those signs. My father was a heroin addict. You see, this is where the story gets personal. I want to tell you about this man, my dad, Raul.
Those good looks, that smile. He could have been whatever he wanted to be. Instead, he shot up.
He was a heroin addict before I was born. He was a heroin addict when I was a child. And ultimately, it got the best of him.
I knew when he was high. I could see the track marks, too. I think my father's drug addiction has really made me who I am today.
You try to be everything that that person couldn't be. He never got to see what became of me and my brother. And I think he would be really proud to see us now.
Philip Seymour Hoffman, the new face of this formidable enemy. Also leaving behind family, children and friends, to grieve heroin's suffocating grip. And Cecilia is here now.
I know it's tough to talk about something so close to your life. Reporter: Yeah. It was strange to see my father on TV. But, Diane, I'm really glad to share this story because I know so many families just like mine out there. Supportive, loving families that learn to live and cope with addiction in our lives. No barriers.
And on the increase, these numbers? Reporter: Yeah. You mention the number of heroin users. The number of heroin-related deaths are up, too. 27,000 people killed because of heroin over the last 10 years. And the power of this drug, we said in the story, is gripping.
This transcript has been automatically generated and may not be 100% accurate.