Preventers are anti inflammatory drugs, or steroids that prevent your airways from becoming inflamed.
They can be inhaled, taken as tablets or, in severe cases, injected.
Inhaled steroids are the most common.
Corticosteroids, steroids for short, prevent the airways from becoming inflamed.
They only work if taken regularly.
Corticosteroids were developed from the natural body hormone cortisol, which is produced by the adrenal gland.
The steroids used to help asthma are extremely effective in stopping the inflammation and swelling of the airways and the accumulation of mucus in the lungs, they damp down the allergic reaction.
While a steroid is a powerful drug, it is safe because you breathe in only a minute amount and, in the prescribed dose, it goes directly to where it is needed, the lungs.
For preventers to be effective, they must be taken regularly, twice a day.
This may seem wonderful to sufferers, but steroids have to be used with care because if you take more than the prescribed dose, or even the prescribed dose over a period, there can be side effects.
Steroids can be inhaled, swallowed in tablet form to relieve an acute attack or to control severe asthma, or injected for an acute severe attack.
Injections are always given by a doctor or nurse.
Inhaled steroids are breathed in using an inhaler puffer, a spacer, a dry powder inhaler or occasionally a nebulizer.
The most widely used drugs are budesonide, beclomethasone and fluticasone.
Unlike bronchodilators, used as relievers, inhaled steroids must be used every day, wven when you are feeling fine, over time they will gradually reduce the inflammation.
If you stop taking them when you are feeling well, your symptoms will gradually return.