See below the description of Shahid Bhai Mani Singh

ਸਜ ਦੁਮਾਲਾ ਸ਼ਸਤਰ ਪਹਿਰੈ I (He would tie a Dumalla on his head and wear Shashtar)

ਨੀਲੰਬਰ, ਗਜ ਸਵਾ ਕਛਹਿਰੈ I (He wore a Blue bana and had a long Kachera – sava gaj)

ਪਕੜ ਦੁਤਾਰਾ ਗਾਵੈ ਬਾਣੀ I (He would hold a saaj and sing Gurbanee)

ਆਸਾ ਵਾਰ ਸੁਖਾਂ ਦੀ ਖਾਣੀ I ੧੮੨ I (He would recite Asaa Vaar and receive the food of peace)

In the same way a similar thing is mentioned in Prachin Panth Prakash about Baba Gurbaksh Singh in his daily routine at Amritvela. It says ਪ੍ਰਾਤ ਉਠੇ ਅਰ ਸੁੱਖਾ ਪੀਵੇ I
Even the modern nihungs would have to admit that this is referring to Naam/Gurbanee here in that context. You would have to be a junkie to be having this as the first thing you do when you get up.

So the terminology of Sukha has sometimes been misinterpreted by some including historians.

However the mention of Sukha as drug also appears in some books, this version of Sukha seems to start appearing in the books of the 19th century. I have not seen it being mentioned in anything credible written in the 18th century, however others can correct me on this. The 19th century seems to be the period of wide spread mal practices in Gurmat and this is well documented.

So initially the use of Sukha may have been introduced for the weaker ones but it has now been turned into some sort of Maryada and a social/recreational drug. It does not serve any purpose in a normal GurSikh’s life.

This is only my analysis on this subject, however I am open to listen to other sensible views from supporters of the Sukha drug on its history and relevance with some proof other than something written in the 19th century.