It also noted that patients who have heart disease are at risk from sexual activity anyway and have to be especially careful if they use Viagra. The new label also warns of the risk of priapism - a prolonged and painful erection - and advises patients to seek prompt medical attention if their erection lasts longer than four hours. Plane crash connection investigated Meanwhile, accident investigators in the United States say they are trying to determine whether Viagra contributed to a plane crash in which the pilot - a Hollywood actor, William Knight - was killed. Mr Knight, 56, who has appeared in films such as Wall Street and Born on the Fourth of July, was killed when his experimental light aircraft hit the water during a difficult manoeuvre. A police spokeswoman has confirmed that officials of the National Transportation Safety Board have asked a medical examiner to determine whether Mr Knight - an experienced pilot - was using Viagra. The American aviation authorities have advised pilots against using the drug within six hours of flying, because its side-effects can include difficulty in distinguishing between blue and green, which are used extensively in airport lighting and cockpit instruments. During testing, Pfizer reported that 3% of men reported visual disturbances that ranged from sensitivity to light to seeing a bluish-green haze.