A selective oxytocin receptor agonist or a pharmaceutically acceptable salt thereof for use in the treatment of Prader-Willi Syndrome, the treatment comprising administering to a patient in need thereof a daily dose of at least 0.5 mg of the selective oxytocin receptor agonist or pharmaceutically acceptable salt thereof, wherein the selective oxytocin receptor agonist or the pharmaceutically acceptable salt thereof is a compound of formula or pharmaceutically acceptable salt thereof: wherein each of W and X, independently, is CH2 and S, provided that W and X are not both CH2; A is an amino acid selected from the group consisted of alanine substituted on the side chain with a 5- or 6-membered heteroaromatic ring; tyrosine; and phenylalanine substituted on the phenyl ring with halogen, C 1-4 alkoxy, C 1-4 alkylhydroxy, C 1-4 alkyl or amino; B is an amino acid selected from the group consisting of isoleucine; and glycine substituted on the a-carbon with C 4-6 cycloalkyl; C is an amino acid selected from the group consisting of proline, optionally substituted on the side chain with hydroxyl, C 1-4 alkoxy, halogen or azide, and proline having its side chain optionally interrupted by a heteroatom and which optionally interrupted side chain is optionally substituted with C 1-4 alkyl; D is an amino acid selected from the group consisting of leucine; homoleucine; isoleucine; and glycine substituted on the a-carbon with C 4-6 cycloalkyl; and E is an amino acid selected from the group consisting of glycine and azaglycine; or wherein the selective oxytocin receptor agonist or the pharmaceutically acceptable salt thereof is a compound of formula or pharmaceutically acceptable salt thereof: wherein n is 0, 1 or 2; p is 0, 1, 2, 3, 4, 5 or 6; R 1 is aryl optionally substituted with at least one OH, F, CI, Br, alkyl, or 0- alkyl substituent; R 2 is R 4, H, alkyl, cycloalkyl, aryl, a 5-membered heteroaromatic ring system, or a 6-membered heteroaromatic ring system; R3 is H or a covalent bond to R 2, when R 2 is R 4, to form a ring structure; R 4 is a C 1-6 alkylene moiety substituted with at least one O-alkyl, S-alkyl or OH substituent; each of W and X, independently, is CH 2 and S, provided that W and X are not both CH 2; alkyl is C 1-6 straight or C 4-8 branched chain alkyl and optionally has at least one hydroxyl substituent; aryl is unsubstituted or substituted phenyl; and cycloalkyl is C 3-6 cycloalkyl and optionally has at least one hydroxyl substituent. each of W and X, independently, is CH2 and S, provided that W and X are not both CH2; A is an amino acid selected from the group consisted of alanine substituted on the side chain with a 5- or 6-membered heteroaromatic ring; tyrosine; and phenylalanine substituted on the phenyl ring with halogen, C 1-4 alkoxy, C 1-4 alkylhydroxy, C 1-4 alkyl or amino; B is an amino acid selected from the group consisting of isoleucine; and glycine substituted on the a-carbon with C 4-6 cycloalkyl; C is an amino acid selected from the group consisting of proline, optionally substituted on the side chain with hydroxyl, C 1-4 alkoxy, halogen or azide, and proline having its side chain optionally interrupted by a heteroatom and which optionally interrupted side chain is optionally substituted with C 1-4 alkyl; D is an amino acid selected from the group consisting of leucine; homoleucine; isoleucine; and glycine substituted on the a-carbon with C 4-6 cycloalkyl; and E is an amino acid selected from the group consisting of glycine and azaglycine; or wherein the selective oxytocin receptor agonist or the pharmaceutically acceptable salt thereof is a compound of formula or pharmaceutically acceptable salt thereof: wherein n is 0, 1 or 2; p is 0, 1, 2, 3, 4, 5 or 6; R 1 is aryl optionally substituted with at least one OH, F, CI, Br, alkyl, or 0- alkyl substituent; R 2 is R 4, H, alkyl, cycloalkyl, aryl, a 5-membered heteroaromatic ring system, or a 6-membered heteroaromatic ring system; R3 is H or a covalent bond to R 2, when R 2 is R 4, to form a ring structure; R 4 is a C 1-6 alkylene moiety substituted with at least one O-alkyl, S-alkyl or OH substituent; each of W and X, independently, is CH 2 and S, provided that W and X are not both CH 2; alkyl is C 1-6 straight or C 4-8 branched chain alkyl and optionally has at least one hydroxyl substituent; aryl is unsubstituted or substituted phenyl; and cycloalkyl is C 3-6 cycloalkyl and optionally has at least one hydroxyl substituent. n is 0, 1 or 2; p is 0, 1, 2, 3, 4, 5 or 6; R 1 is aryl optionally substituted with at least one OH, F, CI, Br, alkyl, or 0- alkyl substituent; R 2 is R 4, H, alkyl, cycloalkyl, aryl, a 5-membered heteroaromatic ring system, or a 6-membered heteroaromatic ring system; R3 is H or a covalent bond to R 2, when R 2 is R 4, to form a ring structure; R 4 is a C 1-6 alkylene moiety substituted with at least one O-alkyl, S-alkyl or OH substituent; each of W and X, independently, is CH 2 and S, provided that W and X are not both CH 2; alkyl is C 1-6 straight or C 4-8 branched chain alkyl and optionally has at least one hydroxyl substituent; aryl is unsubstituted or substituted phenyl; and cycloalkyl is C 3-6 cycloalkyl and optionally has at least one hydroxyl substituent.