The effect of antihistamines can sometimes be altered when they're combined with other substances.

This is known as "interaction" and it's important to try to avoid this whenever possible, as the effects can be unpredictable and potentially dangerous.

First-generation antihistamines

Avoid drinking alcohol when taking first-generation antihistamines because this will increase feelings of drowsiness.

This is the same for other types of medication known to have a sedating effect, such as:

• sleeping tablets
• benzodiazepines, often used to treat anxiety disorders
• tricyclic antidepressants, used to treat a range of mental health conditions, such as depression and obsessive compulsive disorder (OCD), as well as a number of chronic pain conditions

Speak to your GP or pharmacist before taking a first-generation antihistamine if you're taking any of the above medicines.

You shouldn't take a first-generation antihistamine if you're also taking a type of antidepressant known as a monoamine oxidase inhibitor (MAOI). This is because the combination of the two substances can have unpredictable effects.

Second- and third-generation antihistamines

Most second- and third-generation antihistamines don't interact with other medicines. However, the exceptions to this are:

• rupatadine – this can cause unpredictable effects if taken with some types of antibiotics or grapefruit juice
• mizolastine – this can cause unpredictable effects if taken with nifedipine (used to treat high blood pressure), cimetidine (used to treat heartburn) and ciclosporin (often used to treat people who've had an organ transplant)

Cough and cold medicines

Many cough and cold medicines available over the counter at pharmacies contain a mixture of different medications, such as paracetamol, decongestants and antihistamines.

Don't take cough and cold medicines if you've recently taken other antihistamine medication because there's a risk of taking an excess dose.

These types of cough and cold medicines aren't recommended for children under six years old because the risks of treatment are thought to outweigh any benefits.