Brighthaupt also said he saw Smith inject herself twice.
His testimony came during a preliminary hearing to determine if Stern, Eroshevich and Dr. Sandeep Kapoor will stand trial for conspiring to illegally provide Smith with controlled substances.
All three have pleaded not guilty. The defendants are not charged with Smith's death at a Florida hotel in 2007, which was ruled an accidental overdose.
Another potential witness is Larry Birkhead, the father of Smith's 3-year-old daughter, Dannielynn. Defense attorneys were expected to try to block Birkhead's testimony.
Brighthaupt offered surprising new details on the witness stand, saying: "This whole situation - I just have to say something and get it out of me or I'm going to die."
Attorney Steve Sadow, who represents Stern, said he would show on cross-examination that Brighthaupt has reason to testify falsely. He said Brighthaupt was part of a bitter struggle over Smith's estate but didn't elaborate on ways he might cast doubt on Brighthaupt's testimony.
Brighthaupt acknowledged that he and Stern were not on the best of terms. "We've had our differences. Sometimes you think you know somebody and you don't," he testified. "We have disagreements ... I don't wish him no harm."
During his testimony, the bodyguard recalled walking in on Stern as he heated a spoon full of valium tablets over a lighter to liquefy and prepare it for injection.
"She couldn't swallow. They felt it would get into her system faster," he said. "He drew it up into the syringe and he asked me to leave. He didn't want me to see her without her clothes on."
Sadow denied outside court that Stern has injected Smith with medication. He said she was known to take B-12 shots.
The bodyguard was not specific about the timing of the shot and only said it was administered sometime during the stay in Florida.
He also said he saw Eroshevich, a psychiatrist, inject Smith with medication once and deliver a bottle of the sleep syrup chloral hydrate to Smith in the Bahamas after the death of her son, Daniel, who also died of a drug overdose.
He did not specify when the shot was given.
Chloral hydrate was identified in Smith's autopsy as a principle element in her overdose death when combined with other drugs.
In legal arguments, prosecutor Renee Rose offered an explanation to Superior Court Judge Robert J. Perry after he questioned whether the defendants could have controlled how much medication Smith took.
"If you know that a person doesn't have the ability to control, it's like putting a gun in their hand, giving them the ammunition and saying don't pull the trigger," Rose said. "We have a person who was addicted and we have people who over-prescribed to her, and that is why we're here today."