I have had rosacea for several decades, and have tried numerous types of topicals along with regular intake of antibiotics (tetracycline and similar). I recently discovered that vitamin C works very well for my rosacea. I stumbled across this rosacea benefit by accident. I sometimes take vitamin C in the winter to reduce risks of colds. One day I took about 2000 mg of vitamin C when my wife came down with a cold, and I later noticed that the whole next week my face was clearer than it had been in many months. I experimented with it further over the next few months, and for me, taking a daily dose of between 1000 to 2000 mg of vitamin C has a dramatic improvement in the rosacea. I have had rosacea for several decades, and have always taken prescription antibiotics, also I’ve tried several recommended soaps / cleansers, but the vitamin C made a far greater improvement than anything I ever tried in the past. It has a lasting improvement, also... I can skip the vitamin C for a day or two and the skin flushes do not immediately come back.
My regular treatment (now) is to take my daily antibiotics, plus between 1000 to 2000 mg of vitamin C. For the past few months, since I began taking C regularly, my skin has had very little of the flushing, and absolutely none of the occasional disruptive flareups (bumps, thickened skin, pustules) that would so commonly occur every so often. The flushing that does occur is extremely mild compared to what it would do in the past. I have not changed my diet at all recently other than to take the vitamin C.
I have not noted any adverse side effects on myself from this dosage level of C, but of course everyone is different.