I had a crown lengthening done on tooth #18 today and 5 hours later my cheek and lips are still numb. is this normal?

Depends: If your dentist/periodontist used a long acting anesthetic such as Marcaine you may be numb for as long as 8 hours. If they used more a conventional anesthetic such as lidocaine then the numbness should be gone by now. Sometimes damage to the nerve can occur during injection and the effect can last a few months, but that is rare. If this is the case it will eventually get better.

It could be: Different anesthetics will last longer than others. Give it some time and call your dentist if the numbness persists.

Be patient: After 7 or 8 hours, you will most likely feel that the numb sensation is gone. If concerned, see your dentist/surgeon next day.

Speak with DDS: If you have any concerns speak with the DDS and or periodontist that performed the procedure. Need to know the type of local and amount given to ascertain.