Ingrown toenails.

[astarck]

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cut the nails straight, not in a curve.

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Best advice there is to prevent.

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Alternately, dig that [censored] out of there and trim it before it gets worse/infected.

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I agree with this for "experienced" ingrown toenailers.

Ingrown toenails also run in my family, I have had 3-4 (can't remember how many now) cut out, and numerous others I cut out myself before they got too bad. Since this is your first problem I'd go see a doctor as soon as possible. As I'm sure you know by now, these things are very painful.

Once you become more experienced with ingrown toenails you can tell if you are in the process of getting another one and can fix it yourself with proper clipping.