In general, there are three types of drugs that can be targeted for repurposing:

1. Existing generic drugs: A drug is said to be generic when it is no longer affected by patents (i.e. market exclusivity or intellectual property). This means that different companies can make the drug, which normally makes it more widely available and lowers its price.

2. Current branded compounds: These are sold under a licence to treat a specific condition and are protected by intellectual property. These means that they are only produced by a single company.

3. Shelved compounds: These are drugs that have been proved to be safe in humans but did not prove efficacious for a particular disease during their initial development or were deemed not to be worth pursuing. This means that the drugs are shelved e.g. they are retained by the developing company, and never reach market.