Throughout the history of modern medicine, substance-abuse researchers have focused their investigations almost exclusively on men. It wasn't until the 1990s that scientists, prompted by federal funding aimed at enrolling more women in studies, began widely exploring how drug dependence and abuse affects women.

And it turns out that gender differences can be profound.

Women tend to become dependent on drugs more quickly than men, according to the most recent data from the Substance Abuse Mental-Health Services Administration. This is especially the case among those who abuse alcohol, marijuana, and opioids like heroin. Women also find it harder to quit and can be more susceptible than men to relapse, according to Harvard Medical School.

Heroin use has increased dramatically in the United States in the past decade. A new report from the Centers for Disease Control finds heroin use has increased across all income levels and in most age groups.

About 9 percent of those surveyed said heroin would be “fairly or very easily available,” according to the most recent Health and Human Services data. Three in 1,000 Americans report having used heroin in 2013, the most recent year for which data is available. That’s up from two in 1,000 people who reported using heroin a decade ago, according to the Associated Press, and represents 300,000 more heroin users in a span of 10 years. Heroin use among women doubled in that time.