"doubtful efficacy" is a correct and usable phrase in written English.
You can use it when expressing uncertainty about the effectiveness of something, such as in the following example: Although the drug was touted as a miracle cure, its doubtful efficacy was soon apparent.
After a study it became apparent that such an operation could be executed only by the diversion of considerable air support essential to the success of our forces now engaged in decisive operations elsewhere and would in any case be of such doubtful efficacy that it would not warrant the use of our resources.
McCloy responded: After a study it became apparent that such an operation could be executed only by the diversion of considerable air support essential to the success of our forces now engaged in decisive operations elsewhere and would in any case be of such doubtful efficacy that it would not warrant the use of our resources.
Culbreth's 1927 Materia Medica listed D. rotundifolia, D. anglica and D.linearis as being used as stimulants and expectorants, and "of doubtful efficacy" for treating bronchitis, whooping cough, and tuberculosis.
A recent Cochrane systematic review (CD004388) records doubtful efficacy and serious adverse effects.
But the vaccination programs conducted thus far have been of doubtful efficacy.
Such material is liable to be rejected or accepted at very low price causing not only economic loss to the cultivators or collectors of the medicinal plants but also doubtful efficacy or the potency of the raw drug in the alleviation of the human suffering.
doubtful; E, vaccine efficacy; young, nulliparous; NA, not applicable; old, pregnant and lactating.
Second, although patient education as a tool for primary prevention lies within the domain of primary care (and not the hospital), many GPs were doubtful about the usefulness, efficacy and acceptability of the education programmes in the Diagest 3 study.
The other associated therapeutic options, however, are represented by drugs with limited efficacy or a doubtful physiologic rationale, in that they mainly act on body sodium pool, while it is the water pool that is primarily altered.
And safety and efficacy are both doubtful.
Colombian Justice Minister Yesid Reyes Alvarado, nevertheless, said at a news conference held by the U.N. office that the efficacy of spraying was doubtful.
