Absolutely true, but the effect varies from person to person and is only predictable if the sub has taken the medication before. Also, taking prescription medication for "off-label" purposes isn't really advisable.

I would like to know whether the person whose orgasm is being controlled is male or female.
If female, you're probably screwed, unless you're really observant and familiar with her reactions. The best you might hope for would be: 1) Total honesty on her part (ie. co-operating in her own punishment); 2) One of those prescription drugs, though it might make her less horny, or; 3) A topical anaesthetic -- but none of those are great options.
If male, it's a little easier, since it's usually easier to tell when a guy is close to orgasm. In addition to the three female options: 4) A cock-ring might help restrain him, but it can backfire, especially he's a young guy, in which case it may accelerate his orgasm, but keep him hard afterwards, or; 5) The real piece-de-resistance, the squeeze method used to help guys with premature ejaculation to hold back (find an internet piece on sexual dysfunction to explain this more fully).

Does anyone else have any ideas? (I have one for forced multiple O's for women, but it may not qualify as "safe, sane and consensual", so I'm not listing it here. It has worked at least twice, though.)