Merck (MRK) is up about 3% as the market trades into a key late-April U.S. regulatory catalyst: the FDA’s target action date of April 28, 2026 for doravirine/islatravir (a once-daily, oral, two-drug regimen) in adults with HIV-1 infection who are virologically suppressed on antiretroviral therapy. The setup has encouraged risk-on positioning into the decision window as traders price in the chance of an approval that would broaden Merck’s HIV portfolio.

Doravirine/islatravir would add a new fixed-dose option in maintenance therapy, a segment where convenience and tolerability can drive switching dynamics. The stock’s move suggests investors see the April decision as one of the more visible near-term pipeline checkpoints that can help offset longer-term concerns around franchise concentration and post-exclusivity pressure in other areas.

Adding to today’s optimism, Japan granted a global-first approval for Merck’s islatravir/doravirine HIV combination in March 2026, a development that has supported the view that the program can clear major regulatory hurdles. With the U.S. decision expected later in April, traders are treating the remaining wait as a near-term binary that could drive follow-through volatility.

Key swing factors into the April 28 deadline include whether the FDA action arrives on time, the exact label language (including any restrictions tied to prior treatment history), and commercial commentary on anticipated uptake. Any surprise FDA action (approval with limitations, complete response letter, or a delay) could quickly reverse or extend the current rally.