1. Use of a combination of nicotinic acid or nicotinamide with riboflavin for the manufacture of a medicament for systemic administration for the treatment and/or the prophylaxis of pruritus and non-infective disorders involving itching and/or inflammation. 2. Use according to claim 1, wherein said medicament is for the treatment and/or the prophylaxis of pruritus associated with renal insufficiency or failure (i. e. uremic pruritus), and of pruritus forms that are not connected with primary organic affections. 3. Use according to claim 2, wherein said pruritus forms that are not connected with primary organic affections are chosen from the group consisting of itching of aged skin (i. e. senile pruritus), vulvar pruritus, scrotal pruritus and anal pruritus. 4. Use according to claim 1, wherein said medicament is for the treatment and/or the prophylaxis of a disorder chosen from the group consisting of urticaria and/or angioedema, asthma, allergic rhinitis and allergic oculorhinitis. 5. Use according to any one of claims 1-4, wherein said combination consists of nicotinic acid or nicotinamide and riboflavin in a ratio by weight of from 40:1 to 10:1 (nicotinic acid or niacinamide: riboflavin), optionally in a pharmaceutically acceptable vehicle or carrier suitable for systemic administration. 6. Use according to claim 5, wherein said combination consists of nicotinamide and riboflavin in a ratio by weight of 20:1. 7. Use according to any one of claims 1-6 for the manufacture of a medicament for oral or parenteral administration. 8. A method for the treatment and/or the prophylaxis of pruritus and non-infective disorders involving itching and/or inflammation in a subject in need of the same, said method comprising systemically administering to the said subject an effective amount of nicotinic acid or nicotinamide and an effective amount of riboflavin. 9. The method according to claim 8, wherein said medicament is for the treatment and/or the prophylaxis of pruritus associated with renal insufficiency or failure (i. e. uremic pruritus), and of pruritus forms that are not connected with primary organic affections. 10. The method according to claim 9, wherein said pruritus forms that are not connected with primary organic affections are chosen from the group consisting of itching of aged skin (i. e. senile pruritus), vulvar pruritus, scrotal pruritus and anal pruritus. 11. The method according to claim 8, wherein said medicament is for the treatment and/or the prophylaxis of a disorder chosen from the group consisting of urticaria and/or angioedema, asthma, allergic rhinitis and allergic oculorhinitis. 12. The method according to any one of claims 8-11, comprising systemically administering to the said subject an effective amount of a combination of nicotinic acid or nicotinamide and riboflavin, optionally in a pharmaceutically acceptable vehicle or carrier suitable for systemic administration. 13. The method according to claim 12 wherein said combination consists of nicotinic acid or nicotinamide and riboflavin in a ratio by weight of from 40:1 to 10:1 (nicotinic acid or niacinamide: riboflavin). 14. The method according to claim 13, wherein said combination consists of nicotinamide and riboflavin in a ratio by weight of 20:1. 15. The method according to any one of claims 8-14, wherein said systemic administration is by the oral route or by the parenteral route.