Well, generally speaking, Squinting can cause many problems in long term. For example, it can cause headache, the dry eyes, dark circles under your eyes, even the vision sight. Squinting can make your eyes' muscle become stressed. And if this continues for along time, it can also lead to eye stress. And then it can put stress on the eye muscles and the facial and scalp muscles, causing pain and headache. For treat it, you can just do exercises to strengthen your eye muscles. Anyway, just pay more attention to it.
The long term effects of squinting are blurry eyes, headaches and back pain which will not do good for your eyes. You could through the medical care to treat the squinting eyes. Or you could use the good diet to prevent it. At the same time, if you have got this, you could do the surgery to cure the squinting eyes.
There are many bad effects of squinting. First of all, it is easy to recognize it if two eyes are not in the same vision axis, which will definitely impact you looking. I don't what others think. I have a brother who has squinted eyes too. I always feel lost which direction he is looking at. And this makes him hard to find a girlfriend. Besides, there are many negative effects far more serious than this. When one eye is deviating, the other eye needs to take up nearly all the vision function responsibility. The normal eye becomes the dominate eye. Gradually, the squinting eye will lose its function ability, from impaired functionally to totally blindness. And this is hard to discover. It is a slow process and unless the people close the normal eye and use the deviate one to see. Yet there are some ways to prevent it, but you need correct diagnosis and professional support from an oculist. My suggestion is going to hospital at the earliest. Wish you be well soon.