The 6th house is responsible for controlling diseases, health conditions of the native, enemies and rivals of the person, service or workplace of the person, subordinates of the person or servants at home, mental concerns or stress, anxieties, calamities, thefts, and debts. Now, the 6th house is a malefic house, so malefic planets such as Mars, Saturn, Rahu, and Ketu will perform better here. Even Sun can perform here excellently if He is placed in exaltation, own sign, or friendly signs (belonging to Mars specifically). Anyway, Jupiter in the 6th house being a benefic planet may struggle in the house to some extent. However, Jupiter being the most benefic of planets will not suffer much here; the person will still destroy his/her enemies and will have knowledge of how to treat diseases and be in decent health overall, although for this to happen, Jupiter needs to be conjunct with a benefic. Some considerations to note here:
Jupiter in Cancer in the 6th house
This placement is applicable to Aquarius ascendants. Jupiter in Cancer will be exalted in the 6th house which will likely protect the person from much of the misfortune that the house would have otherwise caused to the individual. Such a person will have good finances, but they are likely to spend it carelessly because of Jupiter’s 7th aspect on Capricorn in the 12th house where it debilitates. If the person does not control their spending, they could suffer consequences resulting in mental anxiety and stress, thereby affecting their health.
Jupiter in Sagittarius in the 6th house
This placement is applicable to Cancer ascendants. Sagittarius being a fire sign will give Jupiter the strength it needs to help the native defeat his/her enemies and also build influence over them. Such an individual would be well-known for their valour and capabilities, but they are less likely to be Dharmic, and they are also likely to have issues with overspending or over expenditure.
Jupiter in Pisces in the 6th house
This placement is applicable to Libra ascendants. Pisces is an emotional sign, so it may not be the most optimal sign placement for Jupiter to be in the 6th house. The person will still defeat their enemies but they will have to work hard in their occupation or workplace to build influence and respect. Also, they might overthink their work issues and suffer stress-related health issues as a result of that.