**Cohort or prospective studies**

In cohort studies, drugs are identified and incidences of adverse effects recorded. The weak‐ nesses of these studies include; the relatively small number of patients likely to be recruited, and lack of suitable control groups to assess the background incidence of any apparent adverse reaction noted.

#### **Case control or retrospective studies**

The approach here is to start with the incidence of adverse reaction(s) and then look for the drug and the individuals with symptoms which could be due to an adverse drug reaction. These individuals are screened to see if they had taken the drug. The prevalence of drug taking in the group is then compared with the prevalence in a reference population which did not take the drug. This approach is excellent for validation and assessment of adverse drug effects, but it may not detect new adverse effects. Furthermore, it requires a very large number of patients and is very expensive to undertake hence difficult to justify and organize for every new product.

#### **14.21. Worked examples**
