It's becoming painfully predictable at this point, but the latest setback for GlaxoSmithKline (NYSE:GSK) and partner Ligand Pharmaceuticals (NASDAQ:LGND) still isn't ideal for either. On Friday, the duo announced that the FDA delayed the review of their blood-disorder drug Promacta by as much as three months.

By postponing its review of romiplostim, a rival idiopathic thrombocytopenic purpura (ITP) compound from Amgen (NASDAQ:AMGN), several months ago, the FDA increased its odds of similarly delaying a review of Promacta's New Drug Application. The FDA is now set to rule on Amgen's long-term use ITP drug on July 23.

If the agency takes the full three extra months to make an approval decision on Promacta, its review date should be on or around Sept. 19. Don't assume that date's set in stone; other drugmakers like Cardiome (NASDAQ:CRME) and CV Therapeutics (NASDAQ:CVTX) have proven otherwise.

Ligand needs Promacta approved to juice up its revenue. Its only currently marketed drug is the pain compound Avinza, sold by partner King Pharmaceuticals (NYSE:KG). It receives royalties on Avinza, and if approved, it would receive a 5% to 10% royalty rate on Glaxo's sales of Promacta.

Last month, Promacta was subjected to a relatively positive FDA advisory panel hearing. Although the panel abstained from voting on whether to recommend delaying approval pending more data, it did vote 16-0 that the drug "demonstrated a favorable risk-benefit profile" for short-term ITP treatment.

Promacta's delay is just one of the FDA's many pushed-back review dates this year; aside from some biologics, nearly every new drug has faced postponements. Investors should probably assume at least an additional three-month review period for any New Drug Applications submitted to the agency, given this current harsh regulatory environment.