Like all hanyō, Inuyasha turns completely human once a month (barring one incident in Mount Hakurei due to unique circumstances); in his case, during the new moon. On the night of the new moon his hair turns black, his eyes turn brown, he loses his yokai abilities and attributes, and can only use Tessaiga in its katana form. Only Inuyasha's friends and a few others know his secret as Inuyasha tries to stay hidden during the new moon.
If Inuyasha's life is in danger and the Tessaiga is out of reach, his latent yōkai blood takes over. When transformed, Inuyasha's eyes turn completely red and his pupils become blue slits (similar to those of Sesshomaru and the Inu no Taisho's true yokai forms); ragged purple stripes appear on his cheeks, thin black marks outline his eyes, and his fangs and claws lengthen. In this form, Inuyasha is capable of utilizing his full physical yokai strength, which was enought to destroy Goshinki and Kentomaru in a single hit each and cut throught Ryukotsusei's "rock-hard skin"), and his yoketsu (after cutting it) is strong enough to erode Sesshomaru's Meido Zangetsuha after cutting his own yoketsu with the Tessaiga. However, because yoki is toxic to both humans and hanyos, Inuyasha's human body and mind cannot handle the power he inherited from his father. With every subsequent transformation, Inuyasha's intelligence diminishes, he becomes increasingly irrational and aggressive, and this form becomes easier to trigger and harder to come out of. Not only his mental state, but Inuyasha's hanyo body is also unable to fully cope with the stress of his full yokai strength, meaning that if left unchecked, Inuyasha will continue fighting until his body either gives out or is destroyed by a stronger opponent.
Inuyasha used a shard of the Shikon no Tama to strengthen the Tessaiga's power, an effort which backfired when the impure powers of the shard flowed through Tessaiga, causing Inuyasha to transform. Kagome, seeing Inuyasha's predicament, ran to him and her embrace purified the shard, leaving Inuyasha in a purified version of his yōkai form. In this form, he retained the stripes, eye markings, and pronounced fanges and claws of his yōkai form, but his eyes remained unchanged and he retained his self-awareness. His yōki was increased exponentially, allowing him to enhance the Tessaiga's Kongosoha's barrier-piercing effect with an intensity far superior to previous usage. After the shard was removed from the Tessaiga, Inuyasha reverted to his hanyō self.