My 3.5 mo old daughter's weight is 10 lb-3rd percentile. Eats very less. Dr started cyproheptadine 0.8 ml, is on Pepcid (famotidine) too 0.6 ml twice. Any side effect?

Yes-many. There are side effects to both these drugs--too long to list here. Read the pamphlet or discuss with your pharmacist when you pick up the medications. More importantly it is perfectly fine if a child is at the 3rd percentile as long as they are gaining in ht, wt, and head circumference appropriately. If she is losing weight, that is a serious issue. Check with your pediatrician frequently.