Along with its needed effects, ketotifen may cause some unwanted or undesirable effects. Generally, ketotifen is well tolerated and many people will not experience unwanted effects. Possible unwanted effects include:

• sedation
• weight gain
• increased appetite
• rash
• respiratory infections

Rare:

• sleep disturbance
• headache

Ketotifen is a drug that blocks the release of various chemical messengers in the body that are responsible for producing the symptoms of an asthma episode after a person has been exposed to an allergy producing substance. Ketotifen also works as an antihistamine by blocking the activity of histamine at the body's histamine receptors. By blocking the release of these allergic chemical messengers and by blocking the effect of histamine, ketotifen can prevent the bronchoconstriction and obstructed airways associated with an asthmatic episode. Ketotifen is not a bronchodilator.

It is important that ketotifen users understand that this medication is not a bronchodilator and should not be used as a rescue medication in acute asthmatic episodes.
Ketotifen will only work properly if it is taken on a regular basis. Sporadic use, or only using the medication when experiencing symptoms, will lead to poor symptom control.
Don't expect ketotifen to start working immediately when therapy is started. Improvement may be seen in the first few weeks of starting the medication, but it may take 10 weeks before any significant improvement can be detected.
Never stop using ketotifen abruptly, especially if symptoms have been partially or completely controlled by the medication. Do not adjust your dosage or discontinue ketotifen without the advise of your doctor.
Drowsiness may occur in the early stages of treatment. This effect often disappears with continued use, but caution should be used when preforming activities requiring rapid and precise responses.
Drug Interactions: Ketotifen has relatively few drug interactions, but it is important to inform your doctor and pharmacist of all the prescription and over-the-counter medications your child is taking. Problems have occurred with the following:

• oral antidiabetic medications
• medications with a sedating effect

Use is not recommended in the following situations:

• allergy to ketotifen
• people sensitive to benzoate compounds should not take ketotifen syrup

Use in pregnancy: Safety for use in pregnancy has not yet been established. If you suspect that you might be pregnant, consult your doctor.

Use while breastfeeding: Ketotifen is passed into breast milk. Consult your doctor or pharmacist before use.