Signs Of Weak Mercury In The Horoscope Individuals with a weak Mercury in their horoscope can identify it through the following signs: Relationships with sisters, maternal aunts, and paternal aunts may deteriorate. Even if the person has done nothing wrong, they may face defamation or a negative reputation. Weak Mercury might cause problems with underlying ailments, reducing sexual energy. The person may experience digestive troubles and a weaker digestive system, which could lead to digestive illnesses that cause discomfort.