EMA's Pharmacovigilance Risk Assessment Committee (PRAC) has now confirmed that Zydelig (idelalisib) could cause serious illness, including Pneumocystis jirovecii pneumonia.
But it says that the medicine's benefits outweigh its risks in the treatment of two types of blood cancers: chronic lymphocytic leukaemia (CLL) and follicular lymphoma, which is a form of non-Hodgkin lymphoma.
Both diseases are rare and incurable B cell cancers, and Zydelig is considered to be an important treatment for them. The PRAC has now updated recommendations for use to manage the potential problems.
It says that all patients should be given antibiotics to prevent Pneumocystis jirovecii pneumonia during treatment and for up to 2 to 6 months after treatment has stopped.
They should also be monitored for infection and have regular blood tests for white cell counts - since low counts can increase their risk of infection.
If patients suffer from a generalised infection, they should not begin treatment with Zydelig, PRAC says.
These recommendations will be passed to EMA's Committee for Medicinal Products for Human Use (CHMP) for the adoption of a final position.
During the review, the PRAC had advised physicians not to start Zydelig in patients with previously untreated CLL whose cancer cells have certain genetic mutations (17p deletion or TP53 mutation).
The committee says they can now carry on with Zydelig so long as they cannot take any alternative treatment and that the new precautions are followed.
Zydelig is an oral inhibitor of phosphoinositide 3-kinase (PI3K) delta, a protein that is thought to play a role in the activation, proliferation and viability of B cells.
It was approved in the EU in 2014 and is also available in the US, bringing in sales of $132m last year. The brand has also been predicted to become an $800m-plus product by 2020 for Gilead.
The EMA review came after reports from three trials among patients who received either Zydelig or placebo in addition to other cancer medicines.
These studies did not use the medicine in the same way as currently authorised, but the EMA thinks the risk of infection is relevant to the authorised use.