The 6th house corresponds to an earth sign and is therefore a house of artha (wealth).
Enemies, competitors, jealous people, illness, disease, job. food, appetite, subordinate workers, tenants, debts, miserliness, maternal uncle, cousins, nursing, medical profession, restaurant & catering field, detail & service work.
The 6th house is a dusthana — an evil or harmful house. Planets in the 6th will suffer, as will the houses they rule. Likewise, the ruler of the 6th will harm the house it occupies. But the 6th is the least malicious of the dusthanas, especially since it is also an upachaya house. Upachayas are "growing houses," meaning that any planet posited in such a house grows stronger in time. Therefore any problems caused by involvement with this house may improve or be overcome in time with diligence. A typical example of this can be seen when Mercury (the planet of communication) rules the 2nd (the house of speech) and occupies either the 6th. 8th or 12th. There is sure to be a stammer or speech defect. However, if Mercury is in the 6th, the problem may only be barely noticeable by the time one has reached adulthood. On the other hand, if Mercury occupies the 8th or 12th there is no end to the defect.
Ancient Hindu scriptures declare that if a person's 6th house is well fortified, he will "destroy his enemies". Therefore this house determines the ability to get the better of opponents. It is thus important to have a strong 6th house if one desires to reach the top of any chosen field.
The karakas, or indicators, of the 6th house are Mars and Saturn.
Retrograde 6th lord is good. Enemies and illnesses are weakening, creditors may not demand to repay the debt immediately, agree to deferrals. It also suggests that some enemies and illnesses (or their absence) are associated with the karma of past lives.
Source: «Ancient Hindu Astrology for the Modern Western Astrologer» by James Braha