gistfile1.cs

public class Prescription
{
    public DateTime? CeasedAt { get; set; }
}

gistfile2.cs

public class Prescription
{
    public DateTime? CeasedAt { get; set; }
    public string CeasedBecauseOf { get; set; }
}

gistfile3.cs

public class Prescription
{
    public DateTime? CeasedAt { get; private set; }

    public void Cease()
    {
        if (CeasedAt != null)
            throw new InvalidOperationException("Prescription is already ceased.");

        CeasedAt = DateTime.Now;
    }
}

gistfile4.cs

public class Prescription
{
    public DateTime? CeasedAt { get; private set; }
    public string CeasedBecauseOf { get; private set; }

    public void Cease(string reason)
    {
        if (CeasedAt != null)
            throw new InvalidOperationException("Prescription is already ceased.");

        if (string.IsNullOrEmpty(reason))
            throw new ArgumentNullException("reason");

        CeasedAt = DateTime.Now;
        CeasedBecauseOf = reason;
    }
}