If I buy something OTC that is FSA eligible (e.g. Lactase), can I consider it deductible for a non-FSA taxpayer?

no. you would need a prescription from a doctor. 

Generally, OTCs are not deductible medical expenses.   If a physician prescribes it, it can be deducted however.  

HEALTH FSA AND HRA DEBIT CARDS AND OVER-THE-COUNTER DRUG EXPENSES