De Leon Vanegas Jr. was sentenced Wednesday to 18 years in prison for giving "cheese" heroin to his girlfriend's brother. Fernando Cortez Jr., 15, died hours after snorting the illegal drug last spring.

Mr. Vanegas, 20, must serve half of the sentence before he is eligible for parole. Prosecutors believe Mr. Vanegas is the first person in North Texas convicted of causing a death by giving someone cheese heroin.

Bianca Cortez, now the ex-girlfriend of Mr. Vanegas, testified for the prosecution during the two-day trial. As Criminal District Judge Robert Burns read the jury's sentence Wednesday afternoon, Bianca fought back tears.

"I feel like 18 years is too much," said Bianca, 17. "I'm not saying anyone should feel sorry for De Leon. But my brother was old enough. He knew how to say, 'Yes' and 'No.' "

But Bianca's parents were not as compassionate.

Fernando Cortez Sr., who has become an anti-cheese activist, told reporters that he had hoped for more prison time but would accept the jury's decision.

"He's hurt my family," Mr. Cortez said. "What he's done to me is irreversible. He's taken my boy. I feel for his family. But at least they can visit him. He can get out [of prison] and have a life."

In March 2007, Bianca was living with Mr. Vanegas. He was 19 and she was 16. One day, they picked up Fernando Jr., rented some movies, got some takeout food and stopped by a dope house, according to testimony.

All three snorted lines of cheese heroin, a powder mixture of black tar heroin and cold medicine so named because of its resemblance to grated parmesan cheese.

Bianca said that it was the first time she and Fernando Jr. had used cheese heroin, but that both had used other illegal drugs. That day, Fernando Jr. had been taking Xanax, an anti-depression medication, she testified.

That night, Fernando Jr. went to sleep and never woke up.

"I wish I would have never picked nobody up," Mr. Vanegas testified Wednesday during the punishment phase of his trial.

To the Cortez family, he said, "My heart goes out to you. I'm deeply sorry. I never meant to take anyone from you."

Mr. Vanegas testified that he regularly used cheese heroin and cocaine, and occasionally sold cocaine to family members to support his heroin habit.

He said he started using drugs after his 4-month-old son died in a March 2006 car crash in which Mr. Vanegas was the driver.

Before that, Mr. Vanegas had graduated from high school and been a good kid, his family said.

Prosecutors "went after my son to make an example out of him," an angry De Leon Vanegas Sr. said after the sentence was announced. "Instead of rehab, they are throwing him in the slammer."

Heath Hyde, Mr. Vanegas' attorney, repeatedly said that the Dallas County district attorney's office wanted to use the case to show cheese drug dealers that they will be prosecuted.

"I don't think this was the case for that. This is a good guy who had a rough spot in his life, not a drug dealer," Mr. Hyde said.

But prosecutor David Alex said he believes that Mr. Vanegas was a drug dealer. Regardless, he said, Mr. Vanegas gave a deadly illegal drug to a juvenile.

"But for his actions, that kid would still be alive," Mr. Alex said.