You asked

Can I give adult medicine to my preschooler?

Adult over the counter medications are formulated for adults and are generally not safe for children. Never give a child an adult medication unless your doctor gives you the ok.
Although there are some adult medications that are more than likely safe to give a child, you can never be sure. Manufacturers produce medications with varying strengths, so there is no standard way to determine what adult over the counter medications are safe for children. As an example, one brand of adult paracetamol may have twice the level of the active ingredient, so a small amount could be too much for a child.
Additionally, some adult medicines can contain other active ingredients that can be unsafe for a child to take.
It’s always best to keep children’s over the counter medications on hand, but if you are in a situation where you cannot get a children’s medication, contact a pharmacist or a doctor before you give your child an adult medication.

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