It's generally not a good idea to slather strong dermatological medications directly onto your lips. Remember that your lips are thin and tender, and need protecting.
You may find yourself in a situation where you'll need to apply topical medications, like an anti-acne cream or a cold sore remedy. You may also use facial peels and masks and anti-aging formulas to reduce wrinkles or exfoliate your skin. All of these common products (unlike lip balms) contain drying agents and chemicals designed to remove the top layer of dead skin, or to dry out acne or cold sores.
While you're not applying these formulas directly, they do have a way of traveling to your lips, where they can cause dryness. The best offense is a good defense -- before applying a facial mask or cold sore medication, provide a barrier of protection to your lips. Use something like petroleum jelly or a thick lip balm as the barrier, and you're good to go.