Tramadol is an opioid pain medication used to treat moderate to severe pain. It is usually taken orally and comes in tablet, capsule, and liquid form. It is available both over-the-counter (OTC) and by prescription. It is also available as an injection, which is usually given by a healthcare professional. Common side effects of tramadol include nausea, constipation, dizziness, headache, and drowsiness.