We’re not one to get jealous easily, but when a friend pulls out that green tea clarifying face mask and all we have left in our hands is… basics in TSA-approved sizes. It’s hard not to feel — and maybe even want to be — green.

But let’s not get loopy over a scoop of facial delicacy. There are actually some situations in which you don’t want to try out your friend’s products at all — no matter how radiant their cheeks look.

From your best friend to chill roommate, here are the rules when it comes to sharing skin care.

1. Prescription products

There’s a reason these products are prescribed: they’re curated and measured for an individual only.

Depending on the product and ingredient, prescription-strength skin care may also be stronger than what your skin needs. Meaning, it could damage, dry out, or make your skin more sensitive.

2. Jar based

If you open a jar that has a lot of finger indents going on, you should probably reconsider.

“If you or your friend uses their fingers, they could be leaving behind a number of contaminants from bacteria, other active ingredients from other skin care products, and even skin cells,” says Dr. Caroline Chang, a board-certified dermatologist and founder of the Rhode Island Dermatology Institute.

This also includes bacteria under your nails — or your friend’s nails — and while their skin may be used to battling these microscopic critters, your skin may not be.

The safest bet, if you’re really intrigued and your friend insists? “Make sure a sterile ladle is used to scoop out the product,” Dr. Chang advises.

3. Reusable sponges, makeup brushes, and facial cloths

Unless they’ve been recently washed and freshly sanitized, don’t risk sharing cloths and sponges and definitely skip the sharing if you can tell the tools are still damp.

“Makeup applicators can harbor a number of bacteria from both the skin surface and the bathroom counter such as Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli)” says Dr. Chang. Skip it!

As tempting as it may be to just reach for what’s closest, opt for Q-tips or a paper towel. Especially if you have sensitized or sensitive skin.

If your brain can take a no and spiral it into full-bodied anxiety, we’re with you, friend. While you may look upon their counter or carousel of products with dreams of finding what works for you, it still pays to be extra cautious and before you jump into sharing.

Here are the details to observe before you ask. Plus when hearing a “no” might make more sense.

1. Do you have matching skin types?

Whether it is an antioxidant citrus balm or a sweet floral lotion, the last thing you want is to shake up your skin’s natural defense system. Using the wrong product could lead to reactive skin (drier, oiler, or breakouts).

But, if you both have the same skin type, ask if you could take two drops for a trial. If the product truly worked for your friend, they will be excited in being part of the journey to help your skin.

If you and your friend don’t have the same skin type, don’t worry, there are still some skin care products you can use!

For starters, these are OK to share:

  • micellar water
  • gentle make up removing products
  • low pH or gentle face washes
  • sunscreen

2. Does it actually fit into your normal routine?

No matter how beautiful the bottle looks, assess whether the product makes sense to your routine. Stayovers are probably not the best time to be adventurous and try a random product “just because.”

Is it a toner or eye cream step you usually do but didn’t bring? Or is it a face oil you’ve never tried before as a step?

For many skin care fiends, our skin is a delicate ecosystem accustomed to a certain routine. Dermatologists recommend you avoid randomly using a product because you never know what reaction your skin can have, especially if you have sensitive skin.

3. Is their counter overflowing?

If your friend’s skin care products are spewed all over their bathroom countertop, that could be a sign of an I-don’t-care type of attitude. They’d probably be happy to have you help clear their beautiful clutter, but don’t just casually borrow some. You still should ask.

Another sign is that your friend is chill about sharing other things. And if they have way too many products for the same step — three toners, five serums, four face washes? Well, there’s bound to be one that fits your skin type.

4. Skip if there’s dust and crust

The killer combo of clutter and mess is one you probably want to avoid. If you see dust over the lids or the jar actually audibly cracks when you open it, skip it.

As we mentioned above, open jars can be susceptible to bacteria, so products once opened but no longer in use may be a hygiene nightmare for your face.

If this is the case, switch tactics and see if they’re aware of how this might be negatively affecting their skin. If they are, let them be.

5. Does your friend treasure their skin care?

If your friend reveres their skin care products as prized possessions, then maybe you should continue dreaming about wearing their face mask during movie night.

Mindful behavior down to the droplet is more than just mindful. That product is probably liquid gold to them and it’s a good sign sharing may be out of the picture.

In this case, sigh and adore their beauties, and make a conversation about it instead! Ask them why they love the product and how it changed their game.

The simplest skin care routine involves a cleanser, moisturizer, and sunscreen. You might even notice that going a day without your routine actually does wonders for your skin. (Psst — if that’s the case, it may mean one of your products at-home is doing damage and you may want to consider skin-fasting.)

However, if your skin is in serious need of TLC all the time, don’t be afraid to bring your entire caboose of products with you to that stayover. Your friend, who loves skin care as much as you, will understand.

Anika Nayak is a Florida-based freelance journalist and reporter. She has written for Elite Daily, Teen Vogue, The Washington Post’s Lily, and other outlets. Find her on Twitter or on her website.