Men taking erectile dysfunction drugs should steer clear of a wide range of medicines as well as grapefruit juice and St John’s Wort according to an influential US consumer group.

In a posting on its WorstPills.org website, Public Citizen says that people who take the three approved ED drugs – Pfizer’s Viagra (sildenafil), Eli Lilly’s Cialis (tadalafil) and and GlaxoSmithKline/Bayer’s Levitra (vardenafil) – “should be extremely careful if they take any of 56 other drugs” on a list it has produced. The 56 drugs fall into three categories and the first includes those that can cause “a life-threatening drop in blood pressure” when taken with ED drugs. These include nitrates plus alpha blockers such as Boehringer Ingelheim’s Flomax (tamsulosin) and doxasozin, Pfizer’s off-patent Cardura.

The second group involves drugs that Public Citizen claims may cause ED drug toxicity by decreasing activity of the CYP3A4 enzyme, the most important enzyme for drug metabolism. Treatments on the list here include Novartis’ cancer drug Gleevec (imatinib) and Bristol-Myers Squibb’s HIV drug Reyataz (atazanavir).

The third category covers treatments that speed up the metabolism of the ED drugs, thereby reducing their effectiveness, according to the list. These include Cephalon’s sleep drug Provigil (modafinil) and Actelion’s Tracleer (bosentan) for pulmonary arterial hypertension.

Sidney Wolfe, director of the Health Research Group at Public Citizen, said that the people who take ED drugs “are generally older men, who are more likely to be taking other medications as well”. He added that “it is critical that they be aware of the potential for dangerous interactions”.