Is it permissible to buy and drink milk produced by methods that are harmful to animals, such as injecting them with hormones and unnatural insemination? And is buying it considered participation in sin, or is it forbidden only due to its health ضرر (harm)?

The default rule is that the milk of animals is permissible (halal). Drinking it is prohibited only if the animal has consumed impure substances (najasa), based on the Prophet's (peace be upon him) prohibition of the meat and milk of the jallalah (an animal that feeds on filth). This prohibition is lifted if the animal is fed pure fodder and prevented from eating impure substances. Its milk is also prohibited if it is medically proven to be harmful to health, in order to prevent harm.