How much benadryl (diphenhydramine) can u give 18 month old? weight is 26lbs

None: Generally recommended for children over 6 years. There can be adverse effects in younger children. If you are using it for congestion, then gentle saline and auctioning will help. Keep room cool and not too dry. All the best.

Use with caution: You can try 1/2 teaspoonful or 2.5 ml every 6-8 hrs of Benadryl (diphenhydramine) for his age and size. Be careful due to potential sedation. Claritin liquid is non sedating can be an alternative.