State laws can be more restrictive than federal ones. Some states require e-liquid manufacturers to register with the state, disclose ingredients, or even ban certain flavorings. A few states have tried to cap nicotine strength, and others have banned specific VG/PG ratios under the guise of public health. That's rare, but it happens.

For example, California has its own ingredient disclosure requirements, and some counties have local flavor bans. If you're buying online, the retailer should be following the laws of the state where you live, but that's not always guaranteed. The CDC's state tobacco activities page is a good starting point to see what your state requires.

If you're mixing your own e-liquid, you're stepping into a gray area. The FDA doesn't have a specific rule for DIY mixing, but you're still responsible for labeling if you sell it. And if you're just mixing for personal use, you should still follow basic safety—like using nicotine in a well-ventilated area and storing it away from kids. State laws might require a license for selling nicotine, even in small quantities.