2011 ) created a corpus of posts from several online forums about breast cancer , which later was used to extract potential adverse reactions from the most commonly used drugs to treat this disease : tamoxifen , anastrozole , letrozole and axemestane . The authors collected a lexicon of lay medical terms from websites and databases about drugs and adverse events . The lexicon was extended with the Consumer Health Vocabulary ( CHV ) 5 , a vocabulary closer to the lay terms , which patients usually use to describe their medical experiences . Then , pairs of terms co-occurring within a window of 20 tokens were considered .