Merck & Co Inc said on Tuesday it had got a nod from the U.S. Food and Drug Administration for its already-approved immunotherapy drug Keytruda as a treatment for a type of blood cancer.

The nod from the FDA marks the first approval of the treatment for blood cancer. Keytruda is already approved for treating lung, head and neck cancers, among others.

The drug, administered intravenously, has been approved for use in adults at a fixed dose of 200 mg and in children at a dose of 2 mg/kg for refractory classical Hodgkin lymphoma, a type of cancer that starts in white blood cells.

The drug was approved under the FDA's accelerated approval program which allows for quicker approval of drugs that fill an unmet medical need.

When a drugmaker wins accelerated approval, the company must provide further evidence of the drug's benefit to satisfy the regulator, failing which the approval can be revoked.