The following drugs are very serious drugs with some serious side effects. Always consult your vet professional before using any medications.

Chlorphenaramine

this drug seems to have taken over what bendryl was originally used for.  It is used to help with red, itchy, watery eyes; sneezing; itchy nose or throat; and runny nose caused by allergies, hay fever, and the human cold.

Side effects

Like the other drugs  here,  the use of this drug is not recommended if a dog has:

• asthma, emphysema,
• chronic bronchitis, or other types of lung disease;
• glaucoma
• ulcers
• diabetes
• prostate issues or difficulty urinating (due to an enlarged prostate gland);
• heart disease;
• high blood pressure;
• seizures;
• overactive thyroid gland.

Amitryptiline

Amitriptyline is a tri-cyclic antidepressant. The main use for Amitriptyline is that it affects chemicals in the brain that may be unbalanced in people with depression.

You are not recommended to use this if your dog has had an “SSRI” antidepressant in the past 5 weeks, such as citalopram (Celexa), escitalopram (Lexapro), fluoxetine (Prozac, Sarafem, Symbyax), fluvoxamine (Luvox), paroxetine (Paxil), or sertraline (Zoloft).

The good news is that most dogs that are walked are not clinically depressed.

Bad side effects

• affects mood or behavior changes
• can cause anxiety, panic attacks,
• insomnia or trouble sleeping /restlessness
• can increase impulsive,
• irritable, agitated, hostile, aggressive,
• hyperactive (mentally or physically),
• depression / suicide or hurting yourself.

Not to be used if your dog has:

• history of mental illness or psychosis;
• liver disease / heart disease;
• a history of heart attack, stroke, or seizures;
• diabetes (amitriptyline may raise or lower blood sugar);
• glaucoma;
• any problems with urination.

Avoid exposure to sunlight as a dogs skin sensitivity can be increased and skin burn occurs.

As you can see from these drugs they are potent and the anti histamine effects seems to be a side effect of their primary use.

These forms of drugs have such serious side effects that you should only use with a vet and you have to monitor the doses and your dogs behaviour, not just the condition of their skin and general scratching.

It is clear that these types of drugs are only used as a desperate last measure.

There are many more ‘regular’ antihistamines in the market with far less side effects – so I am not sure why these were even included in the list of ones to take. but I guess they are a great last option if regular human antihistamines over the counter drugs, that you know are safe for your dog don’t work.