A peptide or peptide derivative comprising: (i) WDLYFEIVW;(SEQ ID NO: 1) or (ii) a variant amino acid sequence comprising one, two, three or four L-amino acid substitutions in WDLYFEIVW (SEQ ID NO: 1); or (iii) the retro-inverso variant of the peptide or peptide derivative of either one of parts (i) and (ii), wherein said peptide or peptide derivative has procoagulant activity. A peptide or peptide derivative comprising: (i) an amino acid sequence comprising imfwydcye; or (ii) a variant amino acid sequence comprising one, two, three, four, five or six amino acid substitutions in imfwydcye, wherein said peptide or peptide derivative has procoagulant activity.