Anything applied to your face and skin may cause side effects. Lemons are a citrus fruit, and the juice contains salicylic acid. Fortunately there are few dangers associated with using lemon juice as a topical treatment. But you do need to be aware of a few things.
First, lemon juice is sticky. You’ll be aware of the juice on your skin. If you plan on going outside, it could attract insects. I personally apply it at night before I go to bed. That way I’m not bothered by the feel of it on my skin.
Lemon juice may sting or discolor your skin depending on how sensitive you are. It is recommended that you try applying it in a small patch to your skin and see if you have an adverse reaction. Open sores (form acne or otherwise), cuts and scrapes will be particularly sensitive. Also, you want to avoid getting it in your eyes. It is the equivalent of getting soap in your eye and the associated discomfort.
Lemon juice may dry your skin. Generally it will exfoliate the dead layers of skin and open your pores. However it’s possible that you may experience dry skin as a result of using it.
It may also whiten your skin. Lemon juice is sometimes used as a bleaching agent for scars. It may reduce the redness of scars. If so, it may also lighten your overall skin tone. I personally have not experienced this, and I think its usefulness on scarring is that it exfoliates the skin and that reduces scarring.
Lastly, you are not supposed to be outside in the sun while there is juice on exposed skin. I’ve only ever applied it at night, so I’m not speaking from experience. However, I do recall an invisible ink trick from childhood that involved lemon juice. If you wrote on a piece of paper with the juice, it was invisible. But if you then exposed the paper to heat, the writing would darken and you could read it. You don’t want to risk something similar happening to your face by going out into the sun.