Since none of the ancient Greek and Roman writers have described intoxicating effects of cannabis this leads to conclusion that they were either unaware of them or chose not to use cannabis for intoxication. As a contrast alcohol intoxication was described by many authors, which led some researchers to suggest that cannabis intoxication was less desirable or taken occasionally.
One of the most popular methods of taking cannabis is smoking, practice which reached Europe in the sixteenth century. The only definite description of taking cannabis for pleasure or intoxication in classical literature was described by Herodotus during his travel to Scythia. Another potential reference to cannabis usage could be found in Odyssey, where Helen adds “nepenthe” to the wine of her guests after the siege of Troy. The substance was described as given to her by an Egyptian and some researchers suggest that it was cannabis. “Nepenthe” was given to promote relaxation and discourse rather than heavy intoxication, sedation or psychotomimetic effects. There are suggestions that it was also given to Greek warriors before combat. However, these are only speculations.
There are references in Greek and Roman literature that cannabis was used for medicinal purposes and producing rope and materials. Galen and Ephippus describe how cannabis seeds were used for cooking and eaten as delicacy.
Pliny the Elder (Gaius Plinius Secundus) mentions cannabis in his work “Naturalis Historia”, dated 77 AD. He classifies hemp as belonging to the “fennel class” since like fennel, dill and mallow; hemp plant is a tall, upright, rapidly-growing shrub. Lucius Junius Moderatus Columella describes hemp differently – as “pulse or legume”, together with plants like bean, millet, flax and barley. He also explains that hemp require different times to grow and mature, presumably because this is affected by climate, which varied in the Roman Empire.
There are also references in “Naturalis Historia” about medical use of hemp.
“Hemp at first grew in woods, with a darker and rougher leaf. Its seed is said to make the genitals impotent. The juice from it drives out of the ears the worms and any other creature that has entered them, but at the cost of a headache; so potent is its nature that when poured into water it is said to make it coagulate. And so, drunk in their water, it regulates the bowels of beasts of burden. The root boiled in water eases cramped joints, gout too and similar violent pains. It is applied raw to burns, but is often changed before it gets dry.”
(Pliny, 1950 edn [c])
Dioscorides and Galen mention the ability of cannabis seeds to reduce sexual potency and treat earaches. Pseudo-Apuleius recommends cannabis mixed with nettle seeds and vinegar for cold sores. 
Cannabis was used by Romans for rope in hunting nets, while Greeks made ropes for sailing. There are also evidences that it was used to treat horses. Fresh leaves were used to dress horses’ sores, dried ones against nosebleed. Another usage was as mosquito repellent. 
Read more at Cannabis Chronicles