The gas that creates the foul odor in rotten eggs, hydrogen sulfide, may hold the key to the next blockbuster drug to treat erectile dysfunction, according to a new study. Researchers studied penile tissue discarded by men having male-to-female sex-change operations and found that the tissue expresses hydrogen sulfide; researchers say the gas promotes erections by relaxing the tissue around blood vessels, allowing more blood to flow into the penis. Researchers say the findings raise the possibility of developing an alternative to Viagra by creating a drug that boosts production of the gas.