Sometimes cough drops can contain soothing agents like menthol which if it remains a little bit long in contact with your mucosa may cause a slight 'deadening' of that region. If you had held a cough logenge against your lip for some time this may have caused that numbness. If the numbness goes away in 2-3 hours you need not worry about it. But if it persists beyond that or if it increases in severity or surface area, you should consult your doctor immediately, it may be a sign of some neurological problem.
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