Some mechanisms involve increased metabolism of the drug and adaptive changes at the cellular level. The theory about tolerance to opioids is that there are adaptive changes at the level of receptors for these compounds. Why tolerance would not develop in some patients, but not others is a mystery. However, for various reasons today, I would not use this class of compounds to treat depression. The mechanisms of tolerance to a drug is very complex and not well understood for most drugs.
Some mechanisms involve increased metabolism of the drug and adaptive changes at the cellular level.
The theory about tolerance to opioids is that there are adaptive changes at the level of receptors for these compounds.