1. Want of efficacy.

Nuttall's Standard dictionary of the English language By Nuttall, P.Austin

• inefficency,
• inefficeny,
• ineffeciency,
• inefficieny,
• ineffiency,
• inefficiancy,
• insuficiency,
• ineffciency,
• inneficiency,
• inefficancy,
• inefficientcy,
• ineffieciency,
• ineficiency,
• inefficincy,
• inefficeincy,
• inefficienty,
• inefficience,
• Indeffeciency,
• Indeffiency,
• irondefficiency

• inefficaciously,
• inefficiently,
• inefficacy,
• inefficient,
• inefficacious.

The adjective form of the word "inefficiency" is "inefficient".

The plural of the "inefficiency" can be the "inefficiencies".

The singular of the "inefficiency" can be the "inefficiency".