Editing Adverse effect (section)

===Limitations===
In principle, medical professionals are required to report all adverse effects related to a specific form of therapy. In practice, it is at the discretion of the professional to determine whether a medical event is at all related to the therapy. As a result, routine adverse effects reporting often may not include long-term and subtle effects that may ultimately be attributed to a therapy.<ref>{{cite journal |last1=Bailey |first1=C |last2=Peddie |first2=D |last3=Wickham |first3=ME |last4=Badke |first4=K |last5=Small |first5=SS |last6=Doyle-Waters |first6=MM |last7=Balka |first7=E |last8=Hohl |first8=CM |title=Adverse drug event reporting systems: a systematic review |journal=British Journal of Clinical Pharmacology |date=July 2016 |volume=82 |issue=1 |pages=17–29 |doi=10.1111/bcp.12944 |pmid=27016266 |pmc=4917803}}</ref>

Part of the difficulty is identifying the source of a complaint. A headache in a patient taking medication for [[influenza]] may be caused by the underlying disease or may be an adverse effect of the treatment. In patients with end-stage [[cancer]], death is a very likely outcome and whether the drug is the cause or a bystander is often difficult to discern.{{Citation needed|date=February 2011}}