The side of the box and tube should have instructions, too. If it says "apply every x amount of hours", do that, if it hurts at that time. If it doesn't, and says something like "use as needed" you can use it whenever you need it. Follow the directions on the container.
Also, try using ice or hot packs (depending on the injury) as a non-medicated alternative. Just be careful to not do hot/cold at once or right afterwards; it could cause some damage, or more pain.
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