In medicine, an adverse effect (AE) is a harmful and undesired effect resulting from a medication or intervention and procedures. Some AEs only occur when starting, increasing or discontinuing a treatment; in this case they are a function of dosage or drug levels at the target organs, or they may also be caused by drug interactions. The distinction between adverse and non-adverse effects is a major undertaking when a new drug is developed and tested. Studies are used to define the dosage to be used in human testing (Phase 1) as well as to calculate the maximum admissible daily intake. Imperfections in clinical trials sometimes lead to public health disasters and the withdrawal of the drug from commerce. Surgery may have a number of undesirable or harmful after-effects. Diagnostic procedures may also have AEs, depending much on whether they are invasive, non-invasive or minimally invasive. In clinical trials, a distinction is made between AEs and Serious Adverse Events (SAEs). Generally, any event which causes death, permanent damage, birth defects, or requires hospitalization is considered an SAE.