It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with him) that he used to use the tooth-stick (miswak) while fasting and look in the mirror while in the state of ihram, and he would say: The person in ihram may scratch his head, as long as he does not kill lice or wound the skin of his head.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 10066