The provision of medicinal liquor was a notable loophole in the Prohibition legislation. Pharmacies were allowed to sell medicinal liquor and people were able to buy it on prescription. Famous brands of bourbon, such as Jim Beam, would be sold with warnings on the labels that read "For medicinal purposes only". In actual fact, few people who bought whisky from their friendly neighborhood pharmacies actually had any quantifiable medical issues. (Except alcoholism, of course).
In theory, it was supposed to be difficult for people to get their hands on a prescription that they could use to obtain alcohol. In practice, however, it was ridiculously easy. For the sum of $3 a time, physicians would often write out prescriptions, which the patient could then take to the pharmacy every ten days in order to obtain a pint of liquor. Many large distilleries stayed in business this way right throughout the Prohibition years.
Jews and Catholics were, on the whole, strongly opposed to Prohibition on cultural grounds. Both religions regularly use wine in their services, and so Prohibition was felt to be discriminatory towards these groups. However, under the relevant law, a concession was made whereby 10 gallons of sacramental wine could be made available to each adult per year.
In practice, however, the religious exception was widely open to abuse. Numerous people came forward to obtain the relevant license pretending to be rabbis or priests. As it was difficult to check the relevant credentials of liquor license applicants, it was often too easy for people to get hold of alcohol to which they weren't entitled.
One unintended consequence of this particular loophole of the Prohibition law was the massive increase in congregations at synagogues and Catholic churches. For some people, this was the only way they could legally get a taste of alcohol.