Guru (Jupiter) is the planet of luck or fortune. It is expansive, optimistic energy which brings benefits to whatever planets or houses it associates with. It rules opportunity, prosperity, affluence and money.
It also tells much about a person's deserved benefits earned from activities of the past. This means that what appears in one's life as luck or chance advantages are considered the results of past efforts. So the condition of Guru (as well as the 5th bhava) may be analyzed to derive information about one's past life credit.
Another main attribute of Guru is rulership of religion, philosophy and spiritual affairs. Its influence is more inclined towards appreciation of traditional beliefs, faith and devotion rather than ascetic, occult or mystical tendencies. Guru is capable of bestowing the higher evolutionary qualities of man.
In this vein it is interesting to note that when Guru (Jupiter) and Shani (Saturn) form any mutual aspect (either by conjunction or opposition) a great spiritual nature is the result.
Guru is the chief planetary indicator of children. Therefore if Guru is exalted or very powerful, the person should enjoy a good relationship with his children, and their lives will generally be greatly fortunate. If the Guru is afflicted, the opposite is the case, or there may be very few or no children at all. Of course the 5th bhava must be considered.
Guru is satvic (pure), benefic and masculine. It functions best in Karka (Cancer), its exaltation sign, worst in Makara (Capricorn), where it is fallen.
Guru rules Dhanu (Sagittarius) and Meena (Pisces) and gives very good effects in either of these rashis. Its best placement is in the 4th bhava where it receives dik bala, or directional strength.
If Guru is not dignified or if adversely aspected in the horoscope, it causes restlessness and uncertainty, giving liability to losses through misjudgment, unfortunate speculation, investments and trusts.
In the human physiology, Guru rules the thighs and may cause problems in this area if afflicted. To overcome these tendencies, use care in diet. Dandelion or angelica leaves made into tea is said by physicians to be beneficial for allaying the effects of such disorders, but to get at the cause one must improve the state of mind by banishing worries and partaking of mental relaxation.
In ancient times physicians invoked the aid of Guru for the cure of disease by administering medicine in the "planetary hours" ruled by Guru. Modern physicians, while they have lost the true astrological import, still invoke Jupiter's influence by marking its symbol at the top of their prescriptions, although they have changed it somewhat by making it R instead of as formerly.