The court has heard how McGuire's scheme involved asking for Nurofen Plus at a pharmacy counter and then attempting to pay for it with a card he knew would be declined. In doing this, he created sufficient distraction to discreetly swap the contaminated packet for the fresh one and walk away.
His actions cost manufacturer Reckitt Benckiser £2.4 million when the painkiller was recalled.
McGuire, who went to university at the age of 16, took 32 tablets of the drug each day to feed his secret addiction. But after losing his job he struggled to pay for it.
Instead, the 31-year-old replaced empty packets with the Seroquel he was being prescribed for schizophrenia, and the contaminated packs ended up in the hands of unsuspecting members of the public.
Two men, Peter Letham and Paul Connor, took the anti-psychotic drugs in error, believing them to be Nurofen Plus, and were left feeling unwell.
Mr Letham and Mr Connor swallowed the wrong tablets after Mr Letham's wife, Jacqueline, bought what she thought was a 32-pack of Nurofen Plus from a branch of Boots pharmacy in The Glades Shopping Centre in Bromley, south-east London, on August 21 last year.
The next day Mr Letham, who was employed on a building site, took the packet to work. There he took three tablets and gave two to his colleague Mr Connor, who was suffering neck pain.
Both men soon began to feel unwell, experiencing tiredness and dizziness. They later discovered the drug inside the packet was in fact Seroquel instead of Nurofen Plus.
On July 24 and July 28 last year two other consumers realised the Nurofen Plus they had bought also contained Seroquel instead, but did not swallow any.
The first incident occurred when a woman bought a 32-pack from a Boots pharmacy in London's Victoria station and the second when a man bought a 32-pack in a branch of Boots in Beckenham High Street in south-east London.
In a fourth incident, an assistant at a Beckenham pharmacy found Seroquel tablets inside a Nurofen Plus packet in the store.
McGuire, of Edzell Drive in Glasgow, was tracked down to his landlady's home in Swanley, Kent, on September 23 after the origins of the Seroquel were traced. He admitted his actions and was later charged with causing a public nuisance.
Judge McCreath told McGuire his actions put the building site workers at significant risk and said others may have unwittingly taken the contaminated medication, leaving them similarly vulnerable.
"It needs little imagination to consider the many different circumstances in which that might have happened," he said.
"What if they had been driving a car? Or operating machinery? In short, the risk of potential harm was very high."
He acknowledged McGuire was a man of good character and took steps to overcome his addiction.
But he said that, and the defendant's mental illness, did not prevent him from understanding "at least some of the terrible potential consequences" of his actions.
"Two factors persuade me that you were capable of thinking about what you were doing," he said.
"First, the careful way in which you planned and carried out this enterprise, not on one spontaneous occasion but four times."
Secondly, he said McGuire's acceptance of responsibility did not include "any assertion on your part that you could not understand what the consequences of your actions would be".
Telling McGuire he would serve half his sentence behind bars, minus the days already spent in custody, the judge added: "What you did was potentially very dangerous, caused very substantial economic loss and was something for which you must take responsibility, not in the sense that you intended the consequences of it but because you chose to ignore them."
He said the sentence imposed would have been "very long indeed," were it not for the substantial mitigation offered by McGuire's legal team.
It came after prosecutor Alexandra Felix said he was "terrified" after the health scare hit the headlines and was unsure what to do or who to speak to.
"He did nothing, hoping the matter would resolve itself," she told the court.
"He said he hoped people would notice and return the items to the store. He didn't think about the consequences of his actions or the effect on members of the public.
"He apologised to those who had bought the contaminated medicine."
An overdose of Seroquel can cause coma, tachycardia (rapid heart rate) and hypertension (high blood pressure), she said at the opening of his sentencing on Friday.
"The consequences of the defendant's actions were wide-ranging and spread beyond the actual medical harm suffered by Mr Letham and Mr Connor. (They) must have been to cause fear and alarm among the public as regards the safety of their health, and that can't be minimised," she added.
Lawyer James Hasslacher, defending, said the figure of £2.4 million was an "eye-popping" one for an offence that centred on McGuire saving himself £7.
An extensive investigation was launched when the health scare broke out as "it could not have been perceived that what was occurring was a young man in a very pathetic state of health trying to save himself £7", Mr Hasslacher said.
"He's the most remorseful that one could be, to know the effect of his crime."
He described McGuire as a musically talented and intelligent man "with so much potential", who had begun but not completed a string of degrees.
McGuire had been "reckless and stupid" but his mind had been addled at the time, Mr Hasslacher said.
The defendant nodded quietly as the nature of his addiction was detailed in court this afternoon.
He bowed his head as the judge spoke of the implications of his actions, before waving to a supporter in the public gallery, as he was led from the dock.
