Upvote:2
According to a Catholic Answers Q&A article Is Taking Steroids a Mortal Sin?:
If the steroids are being taken for a legitimate therapeutic purpose, there is no sin. If they are being taken for an immoral or illegal purpose, such as to cheat at sports, then that would be grave matter and one of the conditions necessary for mortal sin (the others being full knowledge and deliberate consent).
Based on natural law principles, the moral determination also needs to take into account:
Every drug, even Tylenol, has potential side effects. The moral question is whether
- the purpose if justified morally
- the risk is offset by the potential gain
- all available information pertinent to risk is sought for from truthful and competent sources (i.e. doctors, not unscrupulous gym body builder coach)
- the drug itself is manufactured ethically (ex. not tainted with misuse of embryos)
- the deliberation is rational, not succumbing to the reverse decision to NOT taking a necessary drug, examples:
- avoiding vaccine thinking it is mostly harmful or not needed
- avoiding life-saving cancer treatment, opting for alternative approach (as a result Steve Jobs died prematurely because he didn't heed medical advice)
Conclusion
Assuming:
- it is not against competition rules
- it is obtained legally (through prescription)
- it is not medically necessary
- it is not needed for one's job (such as professional bodybuilder)
if taking even the prescribed dosage can cause long term health issues for the individual, and if after discernment it is being taken for vanity, then it may be is a venial sin.