I have to take antihistamines/vasoconstrictors regularly to maintain rosacea but i'm worried about my organs handling meds, what to be mindful of?

This sounds like a: very unusual treatment for rosacea. Topical treatments are the mainstay. I have never heard of systemic treatments and chronic antihistamines and vasoconstrictors carry risk as well as tachyphylaxis. I would seek a second opinion preferably from a Dermatologist before continuing with this treatment. Either the diagnosis or the treatment or both is inaccurate.