@ketquest you highlight some important point here.
Not all drugs are equal. Cocaine as a substance, pure or with adulterants, will always be caustic to soft tissue because that is what Cocaine is. Modern medicine would tell you that this ability to reduce blood flow and produce local anesthesia is the drug's most miraculous quality. The first spinal block was performed with Cocaine. It's propensity for limiting blood flow and producing this numbness was a medical miracle at the time of its discovery.
We can use this effect to medical benefit if dosed precisely and not with repeated injections to the same area without allowing the tissue to heal.
You mentioned using Ketamine with no problems via this route. Ketamine does not share these caustic properties (at least to the same degree) with Cocaine, hence we see Ketamine available in literally all dosage forms, from IM injection to intranasal to suppository. What happened with Ketamine doesn't apply to what Cocaine can do. As we're a Harm Reduction society, I feel it's worth mentioning too that Ketamine, though very safe in acute usage, has been demonstrated to potentially cause severe bladder/urological issues in both men and women with chronic administration.
Subcutaneous injection is an awesome answer to Cocaine's short(er) duration of action. Drugs are often administered in this way to allow a gradual release of a drug, for instance insulin. However, as Cocaine is caustic to soft tissue, the idea of pooling a solution of it in your skin for hours is inherently contraindicated.
Don't be fooled by the "nothing bad happened, so nothing bad will happen" logic that we all have fallen for. You can likely get away with something like this once or twice, but not for long.
