Last year I visited gallery of Harappan Civilization at the National Museum, New Delhi. Among the artifacts, I saw a small 'chillum' (hand-held twin pipe) similar to that 'sadhus' (wandering Hindu monks) use in smoking 'charas' (hand-made hashish) and 'ganja' (marijuana). Discussing the history of cannabis, the Wikipedia quotes that charred cannabis seeds were found in a 3rd millenium BC grave in Romania. Submitted with illustration by Vasant Dave
As far as I know, no-one has reported evidence of drugs from the Indus civilization, although there is no reason why they should not have been used.
Cannabis is native to South Asia and is mentioned in the Vedas. Its use was reasonably widespread in the ancient world. Herodotus records that the Scythians has special tents in which people inhaled the fumes from cannabis heated on a tripod. Discoveries in the steppe nomad burials at Pazyryk bear this out. Recently a burial of a man with a sack of cannabis leaves, presumed to be a shaman, was found in Xinjiang province of China. Braziers from the 5th millennium Balkans and from various periods elsewhere in Europe may have been used for heating cannabis or poppy heads.
Poppies, the source of opium, were used in Europe from the Neolithic period onward, and opium was in use in Mesopotamia by the late 3rd millennium BC and by at least the mid 2nd millennium in Egypt; thus the Harappans could also have been introduced to its use. Areca nut and betel leaf, chewed together with lime as paan, are native to South Asia. There is a claim that these were used by the Harappans but I have been unable to track down the evidence on which this claim is based.
The "unicorn" on Harappan seals has a so-called brazier in front of it. There have been various interpretations of this, but general agreement that it is likely to represent a sacred object. It may have been used a brazier to burn incense or drugs such as cannabis or opium, or as a filter to press a liquid (such as soma, the sacred stimulant drink praised in the Vedas, which may have been extracted from ephedra).
Hemp is mentioned as early as the Atharvaveda, but the earliest certain references to cannabis in India are only about 1000 years ago, and the first reference to Tantric sadhus smoking cannabis are in Nagarjuna's Yogaratnamala about 1200 CE. See Dominik Wujastyk's paper "Cannabis in traditional Indian herbal medicine", published in the book "Ayurveda at the crossroads of care and cure", ed. Ana Salema (Lisbon: Universidade Nova de Lisboa, 2002), pp.45--73. (Link
On this basis, Harappan use of cannabis seems unlikely. Does the National Museum artifact have air channels? If yes, it should be chemically examined for any possible remainders of smoke.
We have no evidence if it was or not. It may have occurred, and as far as prevalence is concerned, we do not know.
Update: Nov. 2019, paragraph from J. Bates, Oilseeds, spices, fruits and flavour in the Indus civilisaton
"An example of the over-interpretation of archaeobotanical remains to explain their presence on site can be seen in the discussion of possible stimulants at Kunal (Saraswat and Pokharia, 2002: 133). Although only a single seed of Cannabis sativa, Ephedra sp. and two of Datura stramonium were found at the site, a discussion of their possible role as stimulants and for their “medicinal properties” is carried out, with reference to much later texts, religious practices and non-Indus references. This is not to say that these plants may not have been used for such functions at Indus sites, but that additional information such as assemblage and context is needed to determine this further" (p. 881)