New study examines real-world switching patterns after patients transition from Humira to biosimilars

In a study of nearly 68,000 patients, biosimilar adoption rose after formulary changes, but execution was imperfect: more than 1 in 7 patients switched back to Humira. The results suggest formulary-driven uptake is not fully sticky, implying continued competitive pressure on biosimilar share gains.

Analysis

The investable takeaway is not that biosimilars fail, but that forced substitution is far less durable than modelled. For AbbVie, every incremental patient who migrates back reduces the net price reset investors expected from Humira erosion; for biosimilar makers, the real cost is not launch but retention, because each reversal implies higher salesforce, patient-support, and pharmacy-channel expense just to hold share. That tends to compress margins more than it hurts top-line growth, which is why smaller or less-scaled biosimilar franchises are the most exposed.

Second-order, this is a warning sign for the broader payer playbook: formulary changes can drive a one-time switch, but unless the economic spread is very wide, behavioral friction preserves brand share. That means PBMs and insurers may get less savings than assumed and may respond by demanding deeper rebates, narrower access, or more aggressive step edits in the next contracting cycle. The near-term effect is mostly on the next 1-2 quarters of refill behavior; the 6-18 month effect is more important, because it will determine whether biosimilars become a durable margin pool or a low-ROIC volume trap.

Contrarian view: the market may be overreacting if it interprets switch-back rates as evidence that biosimilars cannot win. The better read is that category penetration is still improving, just with higher churn and slower persistence than bulls assumed. The key falsifier is not the existence of switch-backs, but whether net share gains continue after the next payer re-bid; if biosimilar share stalls or reverses again despite larger discounts, then the entire Humira-equivalent economics thesis weakens materially.

Key Decisions for Investors

• Long ABBV / short XBI for 3-6 months: express the view that legacy biologic franchises retain more pricing power than the market expects while biosimilar economics face retention friction. Target ~1.5-2.0x downside capture to the short leg if the market prices in slower Humira erosion.
• Do not short biosimilars indiscriminately on this headline. Wait for next-quarter net sales and refill data; only press if switch-back rates remain above ~10-15% after payer re-contracting.
• Use AMGN as a tactical underweight vs ABBV rather than an outright short: AMGN’s Humira biosimilar economics are more vulnerable to channel churn and rebate compression than its diversified base implies.
• Set a watch item on payer disclosures and specialty-pharmacy mix: if formulary re-bids narrow the rebate spread but still fail to hold share, the thesis shifts from a launch issue to a structural substitution problem.
• If ABBV trades down on knee-jerk 'biosimilar doom' headlines, buy the dip only on confirmation that net Humira erosion remains orderly; the risk/reward is favorable as long as share loss stays gradual rather than accelerating.