How do we talk about doctors and drugs? Sentiment analysis in forums expressing opinions for medical domain

•Study of how people express their opinion in medical forums.•Opinions about drugs and physicians written in Spanish are analyzed.•Supervised learning and lexicon-based sentiment analysis approaches are applied.•Drug reviews are more difficult to classify than those about physicians.•Linguistic features are analyzed in order to understand the difficulty of the task.•The similarities and differences found are presented and analyzed. The main goal of this study is to examine how people express their opinion in medical forums. We analyze the language used in order to determine the best way to tackle sentiment analysis in this domain. We have applied supervised learning and lexicon-based sentiment analysis approaches over two different corpora extracted from social web. Specifically, we have focused on two aspects: drugs and doctors. We have selected two forums and we have collected corpora for each one: (i) DOS, a Spanish corpus of drug reviews and (ii) COPOS, a Spanish corpus of patients’ opinions about physicians. The classification results show that drug reviews are more difficult to classify than those about physicians. In order to understand the difference in the results, we have studied the linguistic features of both corpora. Although opinions about physicians and drugs are written in most cases by non-professional users, reviews about physicians are characterized by the use of an informal language while reviews about drugs are characterized by a combination of informal language with specific terminology (e.g. adverse effects, drug names) with greater lexical diversity, making the task of sentiment analysis difficult.