Serevent evohaler contains the medicine salmeterol which is a long acting bronchodilator it helps the airways in the lungs to stay open this makes it easier for air to get in and out the effects are usually felt within 10 to 20 minutes and last for 12 hours or more the doctor has prescribed it to help prevent breathing problems these could be caused by asthma taking serevent evohaler regularly will help prevent asthma attacks this also includes asthma brought on by exercise or at nig