There's no evidence to suggest it is very painful. If anything, those that have called emergency services after taking SN have probably freaked out. SN may be somewhat peaceful but you are dying after all.
But there are some resources that claim different reaction in different people.
This is tje post of
@Bentham
Sodium nitrite food poisoning in one family
Forensic Science Medicine and Pathology · October 2018
Cvetković, D., Živković, V., Lukić, V., & Nikolić, S.
A neighbor, who was a butcher, brought some homemade sausages to two spouses (a 70-year-old man and 53-year old woman). They ate some of them for lunch around 13:00. Shortly after lunch, in the afternoon, they started feeling ill and both experienced headache, malaise, dizziness, walking instability, nausea, and vomiting. During the same evening the man experienced chest pain, and shortly after suddenly collapsed and died around 20:00. ... Death was attributed to the exacerbation of hypertensive and ischemic heart disease, resulting from accidental sodium nitrite poisoning.
Two cases of methemoglobinaemia caused by suspected sodium nitrite poisoning
Veterinaria Italiana 44(2):439-53 · April 2008
Matteucci O, Diletti G, Prencipe V, Di Giannatale E, Marconi MM, Migliorati G.
At about 1.00 pm on 22 September 2006, in the province of Teramo, a woman aged 40 (body weight about 70 kg) and her nine-year-old son (body weight about 50 kg) were taken seriously ill 10‐15 min after lunch. The mother reported a cold sensation and presented cyanotic lips, chest constriction and tachycardia. The son presented cyanosis of the lips and hands, chest constriction and loss of consciousness.
Severe Methemoglobinemia due to Sodium Nitrite Poisoning
Case Reports in Emergency Medicine Volume 2016, Article ID 9013816, 3 pages
Kenichi Katabami, Mineji Hayakawa, and Satoshi Gando
A 28-year-old man was brought to our emergency department because of transient loss of consciousness and cyanosis. ... On the second day of admission, the patient was extubated. He then recalled intentionally ingesting approximately 15 g sodium nitrite about 1 hour before ambulance call.
Methemoglobinemia Following Unintentional Ingestion of Sodium Nitrite --- New York, 2002
Morbidity and Mortality Weekly Report (MMWR) July 26, 2002 / 51(29);639-642
On May 16, 2002, Yonkers, New York, emergency personnel were called to a household in which five adults of Middle Eastern descent (three men aged 40, 43, and 44 years and two women aged 60 and 29 years) reported symptoms of dizziness, lightheadedness, and cyanosis almost immediately after sharing a meal. Two of the men also reported vomiting. A sixth person, a man aged 21 years, who did not eat the meal, was asymptomatic.
On arrival, the first responders found the younger woman unresponsive; all others were awake and alert. En route to the hospital, both women had progressive respiratory distress and loss of consciousness and were intubated; the older woman began having seizures.
Fatal methaemoglobinaemia induced by self-poisoning with sodium nitrite
Emerg Med Australas. 2010 Oct;22(5):463-5
Harvey M, Cave G, Chanwai G.
A 76-year-old man collapsed and rapidly developed brady-asystolic cardiac arrest 25 min following self-poisoning with an unknown quantity of crystalline sodium nitrite. On arrival in the ED the patient was asystolic with cardiopulmonary resuscitation in progress.
Deadly meatballs—a near fatal case ofmethaemoglobinaemia
THE NEW ZEALANDMEDICAL JOURNALVol 119 No 1239
Ali Khan, Adrienne Adams, Greg Simmons, Timothy Sutton
Earlier on the evening of admission the patient had eaten microwave-heated meatballs. About an hour later he vomited then lost consciousness.
Fatal methaemoglobinaemia in a dental nurse. A case of sodium nitrite poisoning.
Br J Gen Pract. 1990 Nov; 40(340): 470–471.
W J Gowans
Miss H, a 17 year old dental nurse, was admitted to the casualty department at 18.30 hours after being found at home in an hysterical condition and bright blue. On admission she had central cyanosis, tachycardia and tachypnoea. Her chest was clear and she had a systolic blood pressure of 90 mmHg.
No history was obtainable from her but her boyfriend reported that she had been fit and well earlier in the day and that he had not seen any evidence of drug ingestion at her house.
Fifteen minutes after admission she vomited, aspirated and suffered a respiratory arrest....
by 19.30 hours her blood pressure was unrecordable ... She was certified dead at 20.30 hours, two hours after admission.
IN THE SUMMARY PART THE AUTHOR MENTIONED "TAKING A SINGLE 1 G TABLET OF SODIUM NITRITE" BUT THE REASONING BEHIND IT SEEMS WEAK. see the last paragraph of P. 470.
Seven Cases of Poisoning by Sodium Nitrite
South African Medical Journal 36.1 (1962)
M. L. FREEDMAN
Within 30 minutes of their meal, all the adults experienced palpitation and a sensation of constriction in the throat followed by headache, dizziness and numbness of the finger. There was epigastric discomfort and some nausea but no diarrhoea, vomiting or colic. They all became somewhat drowsy. The headaohe was disturbing and was described as throbbing and bounding.
Accidental poisoning of two laboratory technologists with sodium nitrite.
Clin Chem. 1981 Jun;27(6):1145-6.
J J Aquanno, K M Chan, D N Dietzler
Case one: A 21 -year-old woman was in good health until 30 mm after eating her breakfast in the hematology laboratory of an adjoining pediatric hospital, when she noted the onset of symptoms, which she described as a throbbing roaring sound in her ears. She started work but had to stop because of threatening syncope. She also noticed rapid and strong palpitations as well as a generalized tingling sensation. She induced herself to vomit and presented herself at the Barnes Hospital emergency room. Upon examination, she was noted to have marked cyanosis of the fingertips, lips, and face. A presumptive diagnosis of methemoglobinemia was reinforced when a sample of her blood remained chocolate-brown when ex- posed to air.
Case two: This 34-year-old woman was a co-worker of the other patient. In good health until shortly after eating breakfast in the same laboratory with her, she began to feel weak with diaphoresis and nausea. She also noticed numbness and tingling in her arms and legs. At the Barnes Hospital emergency room, she was found to be short of breath, tachycardic (130 beats/mm), and cyanotic in the arms, legs, lips, and face.
Outbreak of Sodium Nitrite Poisoning
Am J Public Health Nations Health. 1945 Nov; 35(11): 1217–1220.
Morris Greenberg, William B. Birnkrant, and Joseph J. Schiftner
In each instance illness occurred within 5 to 30 minutes after eating breakfast in a cafeteria on Chatham Square. All were between 60 and 80 years of age and all were of the derelict type, living in cheap rooming houses on the Bowery. Seven of those ill lived in the same rooming house; the other four lived in different rooming houses of the same type, and were unacquainted with each other. The clinical picture was similar in all. Within 5 to 30 minutes after breakfast, the involved individuals became dizzy, felt weak, and complained of abdominal cramps. Eight of them vomited and all had diarrhea. Five of them became unconscious shortly after onset of symptoms.
