Backache is a bothersome side effect of tadalafil in some patients, and this mars its efficacy sometimes. If you are taking 20 mg tadalafil, try reducing the dose to 10 mg. This sometimes retains efficacy on erection and eliminates backache. If the backache is still unacceptably severe, use sildenafil. It's equally good. There is no place for the use of injections in your kind of situation, in the year 2007.