Anyone doing this is at high risk for losing a dangerous amount of weight in a very short period of time. So make sure you force yourself to eat. Drinking protein shakes could help. Opiate withdrawal in itself places a lot of strain on your heart and the stimulants increase that. The crashes will be god awful if you do not eat anything all day and your body will heal much more slowly without sleeping/eating.
It's like the flu in a way though. If you don't sleep or eat it really does last much longer. Plus, with heavier withdrawal symptoms it's pretty hard to keep down any food as it is. As horrible as withdrawal is it really is mostly the first two weeks that are unbearable--for some much shorter and others longer depending on severity of using habits. If you're a really heavy abusers of opiates, you're going to need ALOT of crack lol. You're going to have to worry about depression though. The lack of dopamine from opiate cessation is already a problem and stimulants notoriously burn out your dopamine temporarily. I just don't want people to read this thread and then think that this is the best plan to withdraw by. Especially for a chronic relapser. At a certain point using drugs to negate the symptoms of other drugs will really catch up with you in time.