A werewolf is an individual infected with lycanthropy who, during a full moon, will transform into a ferocious, wolf-like creature. Any other night of the month, a werewolf is a perfectly normal human being, and may wield a wand, fly a broom, and mix potions like any other witch or wizard. Werewolves are largely solitary due to the loss of mind when transformed. Wolfsbane potion, intended to ease the transformation of the werewolf and allow him to retain his mind, will not be invented until after the 1970s.

Like vampires, werewolves are created, not made. To become a werewolf, one must be bitten by another werewolf in its transformed state. Lycanthropy is a magical infection rather than a genetic one, and so may not be passed down to the children of werewolves. A werewolf bite, if the victim survives the attack, will always result in infection, though the victim will not begin transformations until the full moon of the following month. There is no cure for lycanthropy.

Though the full moon itself is only technically one day, werewolves will transform on the night to either side of it as well. The days of, and the days leading up to, the full moon will make the werewolf ill in some way — it can affect them as any combination of irritability, fatigue, nausea, and soreness.

A werewolf looks very similar to a normal wolf, though may be a bit larger (particularly if the human is large) and can be distinguished by their pupils, their snout shape, and the tuft of their tail. When transformed, the werewolf will maintain no trace of its human personality, and will instead revert to the mentality of a rabid animal. The werewolf's primary instinct is to hunt, to attack, and the wolf may even turn on itself if other prey is unavailable. The transformation itself is incredibly painful. When the wolf reverts to its human form, it will not usually remember the events of the night before.

It is worth noting, also, that the violence of the transformation, and the resulting physical alterations, will always cause a female werewolf to miscarry in the event that she becomes pregnant. Male werewolves are still able to reproduce as normal when in their human form.

Lycanthropy does not prevent an individual from leading a perfectly normal life when the moon is not full, nor are they any more of a risk to those around them than anyone else when not transformed. That said, a werewolf is considered by most to be something 'dirty' (not unlike vampires), and, if their condition is known, may suffer as a result.

A known werewolf does not have it easy. Their employer may fire them for no other reason; even if they have a sympathetic employer, others may boycott their place of work, particularly if it is a shop. A known werewolf who owns his own business will often see his clientele dry up completely. Their loved ones may face similar treatment if they are seen to be too supportive of the werewolf. Many members of the lower class might find homelessness and financial ruin result from the revelation; even if they manage to hold down a job or find some other source of income, most members of the lower class rent, rather than own, their dwelling, and may get the boot from a landlord.

An upper class man, unless his wealth is dependent on a living relative (such as a father), will get to keep his financial status, but will be socially ruined—as will, likely, his wife and children—and therefore excluded from social functions, see betrothals and business deals dissolve, and be talked about in not-so-hushed whispers from then on. An upper class woman with a husband, father, or brother willing to continue sheltering her will face the same treatment, however should the men in her life cast her out, she will plummet to the lower class. With no skills, no home, and minimal chance at finding a job, she will likely not last long, as most women of the upper classes are not trained in how to fend for themselves.

Children known to be werewolves are not permitted to attend Hogwarts. It would be impossible to keep such a status a secret from the school.

Werewolves who manage to keep their condition a secret, or for whom the knowledge is limited to a few trusted individuals, however, will see only minor changes in their routine. They will have to account for the time spent away from work or social functions around the full moon, and they will need to make provisions to hide themselves when transformed, if they wish to keep their lycanthropy a secret from the outside world. The less visible one's occupation is, and the more money they have to help keep themselves hidden—via modifications of their homes, hiring of trusted assistants to help, etc.—the easier it is to remain accepted by society.