My understanding is that both Forteo and Tymlos, after being taken for the allowed 2 years, are usually followed by some one of the bone maintenance drugs, such a bisphosphonate, to maintain the bone gain.
BTW, You asked about side effects. Eight weeks after I started, my doctored ordered tests for calcium, creatine, and PTH to assess whether the injections might be having an adverse effect; those tests were for me all normal. PTH- parathyroid hormone (often described as a sort of calcium thermostat) when elevated can cause your body to think that you do not have enough calcium resulting in calcium being pulled from bones-- so blood or urine will show excess calcium. Forteo and Tymlos are some type of parathyroid, but if they work as intended, strengthen rather than weaken bones. You might notice a sense of rapid heart rate when you first do the injection, but it goes away quickly; I had it a few times during the first two weeks. Not at all a big deal to me, because I had read that PTH increases heart rate & figured it was just my body acclimating. As I said, I was anxious at the start, but have found that the self injections and the drug were not such a big deal. I sure hope it works to strengthen our spines.