If you've been told you might benefit from a bone-building injection rather than a pill, the conversation can feel like it shifts overnight. You went in expecting to talk about Fosamax or Reclast, and instead your doctor is talking about something called an anabolic agent. As a physician and an osteoporosis patient myself, I want to walk you through what these medications are, why your doctor might recommend one, and what has recently changed about how long you can stay on them.
A quick note on scope before we dive in. There are three FDA-approved anabolic agents for osteoporosis. Two are daily injections that work through the parathyroid hormone pathway: teriparatide (Forteo) and abaloparatide (Tymlos). The third, romosozumab (Evenity), works through a completely different mechanism and has its own treatment schedule, side effect profile, and considerations. Today I want to focus on Forteo and Tymlos, the two daily injection options. Evenity deserves its own dedicated post, and I will write that one next.
Anabolic Versus Antiresorptive: Two Different Jobs
Most osteoporosis medications are antiresorptive. That includes the bisphosphonates (Fosamax, Actonel, Boniva, Reclast) and denosumab (Prolia). They work by slowing down the cells that break down bone, which gives your bone-building cells a chance to catch up.
Anabolic medications work in the opposite direction. They actively stimulate the bone-building cells, called osteoblasts, to lay down new bone tissue. Forteo and Tymlos are both synthetic versions of parathyroid hormone or a closely related fragment, and both have been shown in head-to-head studies to produce larger gains in bone density than bisphosphonates alone.
If bisphosphonates put up scaffolding to protect what you have, anabolic agents are the construction crew adding new floors.
Why a Doctor Might Recommend One
Forteo and Tymlos are not first-line therapy for everyone. Insurance plans and clinical guidelines reserve them for patients at very high fracture risk, or for patients who cannot take or have not responded to a bisphosphonate. The Anthem clinical criteria and the Cigna prior authorization policy reflect this directly. Your doctor will typically consider one of these medications if you fit one of these patterns:
- A T-score of -3.0 or lower
- A history of multiple vertebral fractures, or a recent fragility fracture
- A fracture that occurred while already on bisphosphonate therapy
- A serious side effect from a bisphosphonate, or a medical reason you cannot take one (severe reflux, certain kidney issues, esophageal disorders)
- A FRAX 10-year probability of major osteoporotic fracture above 30% or hip fracture above 4.5%
That last bullet is where FRAX and FRAXplus earn their place in the conversation.
Where FRAX and FRAXplus Fit In
FRAX is a free online calculator developed at the University of Sheffield that estimates your 10-year probability of a hip fracture and a major osteoporotic fracture. It uses your age, sex, weight, height, smoking and alcohol history, prior fractures, parental hip fracture history, glucocorticoid use, rheumatoid arthritis, and (optionally) your femoral neck bone density.
FRAXplus is a newer companion tool that lets clinicians refine that estimate by accounting for things standard FRAX misses, such as recency of a recent fracture, higher-than-average glucocorticoid doses, type 2 diabetes duration, falls in the past year, lumbar spine bone density, hip axis length, and the trabecular bone score. The International Osteoporosis Foundation notes that these adjustments can meaningfully shift probabilities upward in patients whose risk would otherwise be underestimated.
Why does this matter for you? Two reasons. First, the FRAX or FRAXplus number often determines whether your insurance company will approve an anabolic agent. Coverage criteria are written in those exact thresholds. Second, it gives both you and your doctor a clearer, individualized picture of your fracture risk rather than relying on a T-score in isolation. If your T-score is borderline but your FRAX score is high because of falls, family history, or a recent fracture, that is meaningful information.
How They Are Given, and What the Side Effects Look Like
Both medications are self-administered as a daily injection just under the skin of your abdomen or thigh, using a prefilled pen that is similar to an insulin pen. Forteo is given as one injection daily. Tymlos is also one injection daily. The pens are kept refrigerated, and the technique is straightforward to learn.
The daily routine is the trade-off. Most patients adjust within a couple of weeks and settle into a rhythm of injecting at the same time each day, often paired with another habit like morning coffee or brushing teeth.
The most common side effects are mild and tend to improve over time. They include dizziness or lightheadedness when standing (especially after the first dose), nausea, leg cramps, mild headache, and injection-site irritation. Calcium levels can rise temporarily, so your doctor will monitor blood work periodically.
Serious side effects are uncommon. Both medications carry a historical concern about osteosarcoma, a rare bone cancer, based on findings in rats given very high doses for most of their lives. The FDA removed the boxed warning for Forteo in 2020 after long-term real-world studies in humans found no increased risk, and the boxed warning was similarly removed for Tymlos. Real-world osteosarcoma rates in patients on these medications have not differed from background rates in the general population.
What Changed About the 2-Year Limit
For years, both Forteo and Tymlos carried a flat recommendation against cumulative use beyond 2 years in a patient's lifetime. That guidance has softened. The boxed warning is gone, and the current Forteo prescribing information states that use beyond 2 years "should only be considered if a patient remains at or has returned to having a high risk for fracture." The hard ceiling is now a clinical judgment call between you and your doctor, and insurance coverage policies are gradually catching up.
This is meaningful news for patients with very severe osteoporosis who finished a 2-year course years ago and have since lost ground. A second course is now a real conversation rather than an automatic no.
Why a Follow-Up Medication Matters
This is the single most important point I want you to take from this post. The bone you build during a course of Forteo or Tymlos is not permanent on its own. If you stop the anabolic agent and do not start an antiresorptive medication afterward, the new bone you worked so hard to build begins to disappear, sometimes quickly.
The standard sequence, supported by guidelines from the American College of Physicians and other major societies, is anabolic first, antiresorptive second. After 1 to 2 years (or longer, if your situation warrants) of Forteo or Tymlos, your doctor will typically transition you to a bisphosphonate (oral or IV) or, in some cases, denosumab. The follow-up medication locks in the gains. Studies in Endocrinology Today describe this as essential rather than optional. Without the follow-up, the investment of time, daily injections, and cost is largely lost.
Questions Worth Bringing to Your Appointment
If your doctor is considering an anabolic agent for you, here are some questions that tend to make the conversation more productive:
- What is my FRAX score, and have we considered any FRAXplus adjustments that apply to me?
- Is Forteo or Tymlos a better fit for my situation, and why?
- What antiresorptive medication will follow, and how soon after my last injection will I start it?
- How will my insurance handle prior authorization, and is there a patient assistance program if it isn't covered?
- What lab work will we monitor while I'm on treatment?
- Given the updated guidance, would I be a candidate for a longer or repeat course down the road?
Forteo and Tymlos are powerful tools, and the recent shift in long-term guidance opens up real options for patients who were previously told they had used up their lifetime allotment. But they work best as one chapter of a larger plan, paired with a follow-up antiresorptive, with weight-bearing exercise, with adequate calcium and vitamin D, and with a thoughtful approach to fall prevention. None of those things stand alone, and that is genuinely good news. It means there are several places where your effort can make a real difference.
Coming next: a closer look at Evenity (romosozumab), the third anabolic option, including how its mechanism differs, why it carries a cardiovascular caution, and where it fits in the sequence of treatments.