Some experts maintain that over-the-counter medications that combine antihistamines with decongestants are usually ineffective when used in preschool-age children, particularly those under the age of 12.

There also are several side effects to consider before giving a child an antihistamine. These  side effect may vary greatly from the side effects that may occur in adults. Side effects include:

• Drowsiness. Antihistamines can make many children sleepy. For this reason it may be a good idea to give the drug to children at bedtime. This may be less common in children than in adults.
  
  
• Hyperactivity or jitteriness. Some children react to antihistamines with varying degrees of restlessness. Concerned parents should contact their child’s pediatrician to determine the proper dosage or medication type.
  
  
• Seizures. This condition is more likely to occur in children who take antihistamines than in adults. If a child experiences convulsions, parents should alert their child’s physician immediately.
  
  
• Nightmares and irritability. These symptoms may appear in varying degrees in some children. Concerned parents can consult with their child’s pediatrician to determine the proper dosage or medication type for their child.