My 6 month old was 18 lbs too and the pediatrician said we could give 3/4 teaspoon but I thought it was too much so I only gave 1/4 teaspoon and it worked efficiently enough. Better too little than too much. It causes drowsiniess in adults but excitability in kids and it kept my baby awake all night. Benadryl is NOT indicated for children under the age of 2 unless specifically directed by a physician. Please talk to your doctor prior to administering any over the counter anti-histamine, decongestant, or cough suppressant. The CDC 's (Center of Disease Control) comment via LexiComp: The Centers for Disease Control and Prevention (CDC) has released a report concerning the use of cough and cold medications in children <2 years of age. Products containing nasal decongestants (eg, pseudoephedrine), antihistamines (eg, carbinoxamine), cough suppressants (eg, dextromethorphan), and expectorants are often used in this age group. The CDC notes that during 2004 and 2005, ~ 1519 children <2 years of age were seen in emergency departments for adverse effects, including overdose, associated with these medications. During this time period, the deaths of three infants <6 months of age were attributed to cough and cold medications. Safety and efficacy for the use of cough and cold products in children <2 years of age is limited. The FDA notes that there are no approved OTC uses for these products in children <2 years of age. Clinicians are reminded to ask caregivers about the use of OTC products in order to avoid exposure to multiple medications containing the same ingredient. Caregivers are reminded that OTC cough and cold products should not be used in children <2 years of age except under specific direction by their healthcare provider. For additional information, refer to the following CDC website: http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5601a1.htm, Centers for Disease Control, "Infant Deaths Associated with Cough and Cold Medications − Two States, 2005," MMWR Morb Mortal Wkly Rep, 2007, 56(01):1-4.