We definitely aren't saints when it comes to the no-bad habits thing - but when it comes to your makeup routine, there are things you might be doing that you didn’t even know could be harmful to you. Below are some of the most common makeup habits that can cause major damage to your skin and your health. Sharing Makeup How many girls nights have you shared lipgloss or mascara? Unfortunately, sharing makeup with a friend comes the risk of infection according to David Bank, M.D., a dermatologist in Mount Kisco, New York. In fact, in sharing eyeliners and lipsticks, you can run the risk of contracting viral infections such as conjunctivitis and cold sores. Using Expired Makeup Would you drink expired milk? We're going to go with no. So why put expired makeup on your face? You run the risk of getting an infection like pink eye or unwanted skin irritation if you use something that’s expired since products can start to harbour bacteria. Most products have a symbol on them that will tell you how long it’s good for once you’ve opened it. Sleeping with Your Makeup On Everyone has done this at least once. However, no matter how much more convenient it is, you could be waking up to bigger problems in the morning. Sleeping in your makeup can lead to clogged pores but it can also cause eye irritation. When you toss and turn your throughout the night, traces of makeup can migrate into the eye.Eye makeup contains oils and waxes and as the products sit on the eye overnight, those ingredients can block glands, resulting in bumps on the skin around your eye, or even a stye. Getting Lash Extensions Some lash glues contain formaldehyde, which can irritate the eyes. If applied poorly, extensions can cause lashes to become brittle and break off. That being said, getting lash extensions can be perfectly safe as long as you take the right precautions. Make sure you don't overuse extensions, which can lead to hair loss. Also make sure to research where you schedule your appointment to ensure you’re going somewhere safe and doing a patch test to determine if you’re allergic to any extension glue. Applying Eyeliner to Your Waterline Your eyes have their own set of natural bacteria and by using eyeliner pencils, you can introduce foreign bacteria. To avoid infection and clogged oil glands, don’t line your inner rims or waterline [inside your lash line]. In fact, according to a recent study conducted by the University of Waterloo, researchers found that 15 to 30 percent more particles of liner moved into the eye’s protective layer—the tear film—when people applied eyeliner here, as opposed to outside the lash line.