Loperamide have negative effects from rebound constipation. If a suspicion exists of diarrhea associated with organisms that can penetrate the intestinal walls, such as or "Salmonella", loperamide is contraindicated as a primary treatment. Loperamide treatment is not used in symptomatic "C. difficile" infections, as it increases the risk of toxin retention and precipitation of toxic megacolon. Loperamide should be administered with caution to people with liver failure due to reduced first-pass metabolism. Additionally, caution should be used when treating people with advanced HIV, as cases of both viral and bacterial toxic megacolon have been reported. If abdominal distension is noted, therapy
Number needed to treat common (an anticoagulant to prevent heart attack for example), a prospective study may be used. A study which starts with all healthy participants is termed a prospective study, and is in contrast to a retrospective study, in which some participants already have the condition in question. Prospective studies produce much higher quality evidence, but are much more difficult and time-consuming to perform. In the table below: ASCOT-LLA manufacturer-sponsored study addressed the benefit of atorvastatin 10 mg (a cholesterol-lowering drug) in patients with hypertension (high blood pressure) but no previous cardiovascular disease (primary prevention). The trial ran for 3.3 years, and