Viagra, the blockbuster anti-impotence drug, could help men scale ever greater heights. Scientists at Hammersmith Hospital in London have shown that the drug that gives a lift to flagging sex lives can also help people breathe more easily at high altitudes and on mountaineering expeditions where oxygen levels are low. When Professor Martin Wilkins and scientists at the National Center for Cardiology in Kyrgyzstan tested Viagra on people breathing low levels of oxygen, they found that the same enzyme that constricts blood flow to the penis and prevents erections also produced breathlessness at high altitudes by constricting arteries in the lungs. Viagra blocks the action of this enzyme.