Follow the dosing instructions provided by your doctor. Dosing frequency and amounts vary based on the condition being treated. Generally, those being treated for rheumatoid arthritis, psoriatic arthritis, or ankylosing spondylitis need an injection once a week. For children ages two to 17 who have juvenile idiopathic arthritis, injections are given once per week and the amount depends on weight. If you are using Enbrel to treat plaque psoriasis, for the first three months, you will need an injection twice a week roughly every three or four days. After that, you likely will only need an injection once a week.