Just to clarify, the risks associated with testosterone are not acute risks in the same sense as most recreational drugs. You aren't going to end up in the hospital from an overdose or from mixing with other substances (ie. I took too much amphetamine in one dose or mixed amphetamine and a strong depressant).
Chronic use/abuse can lead to long term risks and acute events over time such as heart attack or stoke as CFC has pointed out. Other risks like gynocomastia will be more acute and can happen quickly without proper precautions.