If you’ve spent any time at all in a pharmacy recently, you’ve probably walked past the rows and rows of over-the-counter whitening products, many of which are considerably cheaper than what you’d get from a dental professional.

While the prospect of getting a whiter smile for a low cost may sound appealing, you should know that many of these DIY aligner kits come with hazards if you use them improperly. Here’s how they can be a little bit dangerous.