They don't take the drugs because they have medical issues...they take them because they're abusing them.
Neely doesn't take drugs because she has mental issues..she has mental issues because she's addicted to the drugs.
Once she's a star, however, Neely not only duplicates the egotistical behavior of Lawson, she also falls victim to the eponymous "dolls" (prescription drugs, particularly the barbiturates Seconal and Nembutal and various stimulants). She betrays her husband, Mel Anderson (Martin Milner); her career is shattered by her erratic behavior triggered by her drug abuse, and she is committed to a sanitarium for rehabilitation.
Anne's natural beauty lands her a lucrative job promoting a line of cosmetics in TV commercials and print ads. She also falls under the allure of drugs to escape her doomed relationship with cad Lyon (Burke), who has an affair with her erstwhile friend, Neely.
This addiction/drug abuse is the basis for the whole movie.