Heroin prepared for the European market is insoluble in water and usually requires the addition of an acid such as citric acid or ascorbic acid (Vitamin C) powder to dissolve the drug. Due to the dangers from using lemon juice or vinegar to acidify the solution, packets of citric acid and Vitamin C powder are available at needle exchanges in Europe. In the U.S., vinegar and lemon juice are used to shoot crack cocaine.