Benadryl (generic name diphenhydramine) is usually considered safe to take while pregnant and is commonly used as a sedative and antihistamine for treating minor cold and flu symptoms such as runny/stuffy nose, congestion, sneezing, watery eyes, nausea as well as allergy symptoms like rashes, itching and hives . It may also be used for management of minor anxiety and as a sleep aid by pregnant women .
It is never advisable to use Benadryl in pregnancy without consulting your doctor to assess the potential risks .
Category B – Diphenhydramine does cross the placenta but animal studies show little evidence of harmful effects on the fetus. However, there is a lack of data regarding its effects on a human pregnancy .
Doctors usually recommend taking a 25mg to 50mg oral dose once every six hours  in the first trimester, while the dosage should not generally exceed 25mg in six hours during the last two trimesters [6, 7].
Use of topical Benadryl is considered safe in pregnancy as there is little evidence of harm to the fetus. However, there is inadequate data regarding whether the diphenhydramine in the ointment form crosses the placenta . Expert medical guidance is recommended to assess chances of complications.
Drowsiness is the most common yet harmless side effect on this drug.
In case of overdose: One case study shows oxytocin-like effects of the drug in case of high dosages and overdose. Moreover, it may have certain additional adverse effects when used by pregnant women, which are less likely to occur in non-pregnant patients .
Fetal morbidity: One study showed the use of Benadryl along with Restoril (temazepam) to significantly increase the chances of fetal morbidity.
Risk of birth defects: Certain case reports have shown a possible association between the use of diphenhydramine in the first trimester and an increased risk of cleft lip or palate in the baby .
Withdrawal symptoms: In another case study, daily use of 150 mg Benadryl by the mother led to withdrawal symptoms in the baby.
Occasional use has not been associated with any serious adverse effects in the nursing infant . However, prolonged use is contraindicated as the drug is excreted in the breast milk when taken by breastfeeding women . Long term use or larger doses have been shown to decrease the breast milk supply of have some negative effects on the infant (e.g. irritability and lethargy).