Antihistamine shots are prescription medications that are used for the rapid medical treatment of conditions such as:

• Allergic reactions (a severe allergy to a substance)
• Anxiety
• Nausea
• Vomiting
• Motion sickness
• To induce sedation when injected into a patient

Antihistamine shots also are used to alleviate extra pyramidal symptoms of antipsychotic medications.

Histamine is a chemical that causes many signs and symptoms of allergy. Histamine is released from histamine-storing cells (mast cells) and attaches to other cells that have receptors for histamine on their surfaces. Histamine stimulates the cells to release chemicals that produce effects that we associate with allergy. Antihistamines blocks histamine receptors and thus prevent activation of cells with histamine receptors by histamine.

Examples of the brand and generic names for this type of medication include:

• diphenhydramine (Benadryl)
• promethazine (Phenergan)
• hydroxyzine hydrochloride (Restall, Vistacot)
• dimenhydrinate (Dramamine, Driminate)

The formulation of this drug is available by prescription only to treat a patient with a specific condition. It is not available over-the-counter (OTC). Other formulations of antihistamine drugs are available over-the-counter that treat medical conditions such as:

• Mild allergic reactions
• Itching
• Mild allergy symptoms
• Allergic rhinitis
• Runny nose

Examples of OTC medicine available over-the-counter include:

• diphenhydramine (Benadryl)
• certirizine (Zyrtec)
• loratadine (Claritin)
• fexofenadine (Allegra)
• desloratadine (Clarinex)
• loratadine ODT (Alavert, Tavist ND)

The common side effects of this type of medication include:

• Dizziness
• Drowsiness (sleepiness)
• Dry mouth
• Blurred vision
• Urinary retention
• Increased or decreased blood pressure
• Headache
• Abnormal heart rate
• Nausea
• Trouble breathing
• Tiredness (fatigue)
• Weakness

Antihistamine injections have additive effects if used with alcohol or other central nervous system depressant medications such as:

• alprazolam (Xanax)
• clonazepam (Klonopin)
• diazepam (Valium)
• zolpidem (Ambien)
• zaleplon (Sonata)
• phenobarbital
• butalbital/acetaminophen/caffeine (Fioricet)
• codeine
• morphine (MSIR)
• hydromorphone (Dilaudid)
• buprenorphine (Buprenex)
• azelastine (Astelin)

Effects of antihistamines are prolonged if combined with monoamine oxidase inhibitors (MAOIs) such as:

• phenelzine (Nardil)
• selegiline (Zelapar, Emsam, and Eldepryl)
• tranylcypromine (Parnate)
• procarbazine (Matulane)
• rasagiline (Azilect)
• isocarboxazid (Marplan).

Antihistamine injections should be used with caution if the patient under medical care and receiving treatment with an MAOI drug.

• Prescription antihistamine injections are available in injection form.
• Diphenhydramine, promethazine, and dimenhydrinate are administered intramuscularly and intravenously.
• Hydroxyzine hydrochloride is administered only intramuscularly.
• Antihistamine injections are not recommended for administration via subcutaneous route.

• FDA lists diphenhydramine and dimenhydrinate as Pregnancy Category B and promethazine as Pregnancy Category C. Hydroxyzine does not have a Pregnancy Category. These categories mean that the safe and effective use of any antihistamine injection drug is not established in during pregnancy; therefore, if a patient is pregnant, medical treatment with this drug should be used only if clearly needed.
• It is not known whether an injected antihistamine drug enters breast milk; therefore, patients who are pregnant should seek medical advise and use caution before treatment with a prescription antihistamine injection medication in women who are breastfeeding.