Antihistamines that are taken by mouth (oral) work better than those that are applied directly to the skin (topical) because a pill or capsule contains a specific dose of medicine. The dose in a cream or ointment depends on how much is applied at one time and is harder to control. Too much antihistamine absorbed through the skin can be toxic, especially to children. Don't give any antihistamines to your child unless you've checked with the doctor first. The use of cream or ointment antihistamines is not reliable and not recommended.
Do not take oral antihistamines when you are driving, are operating machinery, or need to be alert because they can make you sleepy.
Use caution if you have other health problems, such as glaucoma, epilepsy, or an enlarged prostate. Antihistamines can cause your other health problems to get worse and also may interact with other medicines, such as antidepressants, sedatives, and tranquilizers. Read the package carefully, and ask your pharmacist or doctor to help you choose an antihistamine that will not cause problems.
Antihistamines are often combined with a decongestant in one product. These medicines may not be safe for young children or for people who have certain health problems. Read and follow all instructions on the label.
Antihistamines may make young children sleepy or may stimulate the nervous system, causing hyperactivity. Don't give antihistamines to your child unless you've checked with the doctor first.
In rare cases, diphenhydramine (especially forms of it that are applied to the skin) can cause severe side effects in children, such as hallucinations, tremors, and coma.