Nancy Reagan broke down and cried yesterday as she read aloud to an audience of several hundred pharmacists a teen-age girl's dramatic letter telling how drugs "ruined" her life.

In one passage, which Mrs. Reagan described as "a little bit raw and hard to take," the girl, now 16, told of having sex with a 55-year-old man for $500 worth of drugs. She also said that she was once pregnant, although her parents never knew it, and that because of drugs she had the baby four months early.

"The baby's arm was at its leg and its ear was at its cheek. The baby died," wrote the girl.

Mrs. Reagan identified the girl only as "Joanie" who lives in a small midwestern town and attends a girls' school. Her letter told how she became a drug addict in 1980 at age 13.

"I got to the point where I was high all the time," the girl wrote. "When I'd wake up I'd either shoot up or pop pills and then take a few joints, usually six. For me, drugs were the escape from reality and I never wanted to stay in the real world. To top it off I was adopted as a baby and when I found out I was different I never wanted to be in the real world . . ."

Mrs. Reagan read the letter at a Washington Hilton luncheon given by Johnson & Johnson to announce "Pharmacists Against Drug Abuse" (PADA), a national campaign enlisting neighborhood pharmacists to educate families about drugs. McNeil Pharmaceutical, a Johnson & Johnson subsidiary that makes Tylenol, will launch the campaign later this year in cooperation with the volunteer agency ACTION.

The first lady explained that she decided to read the letter in its entirety "rather than give you statistics or speak in generalities because I'd like to make it real for you."

She said she receives "hundreds" of letters like it "and they are really only the tip of the iceberg."

At one point Mrs. Reagan excused herself--"I'm sorry," she said, overcome by tears and unable to continue. Then recovering, she said, "You here today can help prevent other children from going through what Joanie did. If you could reach one child, wouldn't it be worth it?"

Sheila Tate, the first lady's press secretary, said later that the three-page handwritten letter, which was signed and carried a return address, arrived at the White House about a week ago. Mrs. Reagan answered it the same day, telling the girl her letter had "touched my heart" and asking her to "write again in a month or two to let me know how you are doing."

Tate said Mrs. Reagan wanted to establish communication and to make the girl, whom her staff gave the fictitious name "Joanie," feel better about herself.

Joining Johnson & Johnson board chairman James E. Burke and McNeil Pharmaceutical president Jack E. O'Brien at yesterday's head table was television star Michael Landon, who told of his own daughter's battle with drugs.

"I look back and I am honest with myself. I know she already had a basis for that problem, but like a lot of parents I think I just wanted to try to avoid it," said Landon. "Unfortunately, most of us don't want to face the fact that our kid is into drugs . . ."

Landon, who has filmed a public service announcement for PADA's new campaign, said that eventually parents get to the point where they don't know what to do anymore.

"I thought I could do it with love and understanding and being with her. But that's not strong enough to do it," he said. "They're going to use you, they're drug addicts. They're going to lie to you and you're going to want to believe them desperately."

In the end, he said, a child has to be made to realize that the body being ruined is his own and that until he takes "responsibility" for it, parents "won't get anyplace."