The problem with speedballing is that the cocaine acts as a CNS stimulant while the heroin acts as a strong CNS depressant, but cocaine wears off in like 15-20 minutes and heroin takes considerably longer, which leads to many people overdosing by respiratory depression when the cocaine wears off.
Lots of people use heroin to come down off of cocaine. I'm not condoning it or saying that it's safe or a good idea, but the biggest danger is with speedballing, which implies your taking both of the substances together at the same time. But if the cocaine has already worn off and your coming down, just be careful and instead of doing your normal full-sized shot of heroin, do a test shot and then try a modest dose. You can always shoot more heroin, but once it's in, it's in to stay for good or for bad, so be smart and just titrate your heroin until you're comfortable. DO NOT ATTEMPT TO DO ONE BIG/NORMAL SHOT OF HEROIN IF YOU FEEL YOU STILL HAVE COCAINE IN YOUR SYSTEM. AFTER DOING A TEST SHOT TO GAUGE THE POTENCY OF THE HEROIN YOU HAVE, DIVIDE YOUR NORMAL DOSE OF HEROIN INTO TWO SHOTS.
For example, if you normally shoot 2 bags of heroin, after doing a test shot for potency, with two syringes prep 1 bag per syringe, and see how you react to 1 bag, giving yourself like at least an hour before shooting the second. This should give you time for a lot of cocaine to leave your system and it will give you time to properly dispose of the two single-use syringes you used.