Curcumin is more than just a yellow spice (turmeric). It's a potent anti-inflammatory. And that is why it is useful for rosacea.
The problem with it, though, is that it is poorly absorbed. You can buy all the supplements out there, and some will do nothing for you because they aren't formulated well enough for decent absorption. Or you don't eat enough fat when you take it to absorb it. Fat aids the absorption, so I've even eaten a tablespoon of coconut oil with it! Anything with some fat in it will help--whole milk, nuts, red meat. Another thing that helps the bioavailability is the addition of black pepper, or piperine. Pipeline blocks the metabolism of curcumin so that it's available in the body longer.
Also, evidently, heat increases the absorption. I haven't tried this, but perhaps drinking hot tea after taking your curcumin supplement with plenty of cream in it would help. Then you get both heat and fat.
If you want to find a good supplement, one place to look at quality reviews is Consumer Labs. You have to subscribe to read all the reviews, but it's about $39 a year. That's for those of us who really want to dig deep into what we're buying. Otherwise, I can recommend Jarrow curcumin 95 as a good product that has high standards.
The active components are curcuminoids, which are anti-inflammatory and anti-cancer. It's especially useful in dermatological inflammation, which is exactly what we're looking for!
It essentially has no side effects or overdose potential as long as it's a good quality supplement! People's chemist makes a good one combined with other very useful ingredients, for example.
While this is great for rosacea, it has so many other health benefits that pretty much everyone should take it. In my opinion, it's like astaxanthin--everyone can benefit and it has few downsides.
Will it cure rosacea? Not if you don't also do something about how much sugar you're eating because you can't keep pouring on inflammation and expect this to magically wipe it out. But if you take this and also cut out sugar, you'll probably find your breakouts quickly improve and reduce in frequency.
Curcumin can stimulate contractions or even a period, so this is not safe in pregnancy or if you're trying to get pregnant. But the good news is that many women find their rosacea improves a lot in pregnancy! Mine did! I even forgot I ever had it. There are other skin problems to worry about, like strange pigmentation.
Another option is to try it topically on a breakout. You can make a paste with an oil, like jojoba, and some turmeric and apply it to your affected areas. Don't forget that it is a bright yellow and it will stain clothing or bedding. I'd say to leave it on for 15 minutes or so and wash it off.