Heparin, also known as unfractionated heparin (UFH), is a medication and naturally occurring glycosaminoglycan. As a medication it is used as an anticoagulant (blood thinner). Specifically it is also used in the treatment of heart attacks and unstable angina. It is given by injection into a vein or under the skin. Other uses include inside test tubes and kidney dialysis machines.Common side effects include bleeding, pain at the injection site, and low blood platelets. Serious side effects include heparin-induced thrombocytopenia. Greater care is needed in those with poor kidney function. Heparin appears to be relatively safe for use during pregnancy and breastfeeding. Heparin is produced by basophils and mast cells in all mammals.The discovery of heparin was announced in 1916. It is on the World Health Organization's List of Essential Medicines. A fractionated version of heparin, known as low molecular weight heparin, is also available.