I saw this image on TV tropes:
Which anime is it taken from?
This image is from the third game in the In'youchuu eroge series, In'youchuu Etsu ~Kairaku Henka Taimaroku~. It does have an OVA adaptation, but I've linked to the first OVA in the series rather than this one as the image for this one is not work-safe. If you do intend to search for information regarding this, keep in mind that this series is literally pornography and that most of the content related to it would not be appropriate to post here.
VNDB has the following description of the game:
One day, Yamato, Mikoto and Takeru visit a village in the mountain for the training to be monster hunters. They have various hard trainings and finally become the monster hunters. But there is something wrong with Yamato's body.... His body gradually changes into a monster.... What is happening to his body? Who can stop it? To save Yamato, Mikoto and Takeru make every effort. What is a secret of Yamato's body? What will happen to Mikoto and Takeru?
The characters in the image are Yamato and Mikoto. Yamato's body has a sealed demon inside him, which is released at this particular moment in the story. This image is an image of Yamato from the OVA: